Add plugin documentation page (AcademySoftwareFoundation#1703)

Signed-off-by: Bryce Gattis <brycegattis@yahoo.com>

docs/source/index.rst

@@ -31,6 +31,7 @@ Welcome to rez's documentation!
    managing_packages
    caching
    pip
+   plugins
 
 .. toctree::
    :maxdepth: 2

docs/source/plugins.rst

@@ -0,0 +1,140 @@
+=======
+Plugins
+=======
+
+Rez is designed around the concept of plugins. Plugins can be used to extend rez's functionalities without modifying any of rez's source code.
+
+Rez comes with built-in plugins that are located at :gh-rez:`src/rezplugins`. New plugins are encouraged to be developed out-of-tree (outside rez).
+
+List installed plugins
+======================
+
+Currently installed plugins can be queried by running :option:`rez -i`
+
+.. code-block:: console
+
+   $ rez -i
+
+   Rez 2.113.0
+
+   PLUGIN TYPE         NAME        DESCRIPTION                                        STATUS
+   -----------         ----        -----------                                        ------
+   build process       local       Builds packages on local host                      loaded
+   build process       remote      Builds packages on remote hosts                    loaded
+   build system        cmake       CMake-based build system                           loaded
+   build system        custom      Package-defined build command                      loaded
+   build system        make        Make-based build system                            loaded
+   package repository  filesystem  Filesystem-based package repository                loaded
+   package repository  memory      In-memory package repository                       loaded
+   release hook        amqp        Publishes a message to the broker.                 loaded
+   release hook        command     Executes pre- and post-release shell commands      loaded
+   release hook        emailer     Sends a post-release email                         loaded
+   release vcs         git         Git version control                                loaded
+   release vcs         hg          Mercurial version control                          loaded
+   release vcs         stub        Stub version control system, for testing purposes  loaded
+   release vcs         svn                                                            FAILED: No module named 'pysvn'
+   shell               cmd         Windows Command Prompt (DOS) shell.                loaded
+   shell               gitbash     Git Bash (for Windows) shell                       loaded
+   shell               powershell  Windows PowerShell 5                               loaded
+   shell               pwsh        PowerShell Core 6+                                 loaded
+
+
+Configuring plugins
+===================
+
+Plugins can be configured by adding a ``plugins`` key to your ``rezconfig.py``
+like this:
+
+.. code-block:: python
+
+   plugins = {
+       "filesystem": {}
+   }
+
+Existing plugin types
+=====================
+
+- :gh-rez:`build_process <src/rezplugins/build_process>`
+- :gh-rez:`build_system <src/rezplugins/build_system>`
+- :gh-rez:`command <src/rezplugins/command>`
+- :gh-rez:`package_repository <src/rezplugins/package_repository>`
+- :gh-rez:`release_hook <src/rezplugins/release_hook>`
+- :gh-rez:`release_vcs <src/rezplugins/release_vcs>`
+- :gh-rez:`shell <src/rezplugins/shell>`
+
+Developing your own plugin
+==========================
+
+Rez plugins require a specific folder structure as follows:
+
+.. code-block:: text
+
+    /plugin_type
+        /__init__.py (adds plugin path to rez)
+        /rezconfig.py (defines configuration settings for your plugin)
+        /plugin_file1.py (your plugin file)
+        /plugin_file2.py (your plugin file)
+        etc.
+
+To make your plugin available to rez, you can install them directly under
+``src/rezplugins`` (that's called a namespace package) or you can add
+the path to :envvar:`REZ_PLUGIN_PATH`.
+
+Registering subcommands
+-----------------------
+
+Optionally, plugins can provide new ``rez`` subcommands.
+
+To register a plugin and expose a new subcommand, the plugin module:
+
+- MUST have a module-level docstring (used as the command help)
+- MUST provide a `setup_parser()` function
+- MUST provide a `command()` function
+- MUST provide a `register_plugin()` function
+- SHOULD have a module-level attribute `command_behavior`
+
+For example, a plugin named 'foo' and this is the ``foo.py`` in the plugin type
+root directory:
+
+.. code-block:: python
+   :caption: foo.py
+
+   """The docstring for command help, this is required."""
+   from rez.command import Command
+
+   command_behavior = {
+       "hidden": False,   # optional: bool
+       "arg_mode": None,  # optional: None, "passthrough", "grouped"
+   }
+
+   def setup_parser(parser, completions=False):
+       parser.add_argument("--hello", ...)
+
+   def command(opts, parser=None, extra_arg_groups=None):
+       if opts.hello:
+           print("world")
+
+   class CommandFoo(Command):
+       # This is where you declare the settings the plugin accepts.
+       schema_dict = {
+           "str_option": str,
+           "int_option": int,
+           ...
+       }
+       @classmethod
+       def name(cls):
+           return "foo"
+
+   def register_plugin():
+       return CommandFoo
+
+All new plugin types must define an ``__init__.py`` in their root directory
+so that the plugin manager will find them.
+
+.. code-block:: python
+   :caption: __init__.py
+
+    from rez.plugin_managers import extend_path
+    __path__ = extend_path(__path__, __name__)
+
+