various doc updates

[cs01]

• I have added an entry to docs/changelog.md

Summary of changes

• Add example using --pip-args (from @dalito, Docs: Add example for using "--pip-args" #772)
• Add PIPX_HOME and PIPX_BIN_DIR example (@alextremblay, Update installation.md #755)
• Move CHANGELOG.md and CONTRIBUTING.md to top level, and symlink from docs dir. Previously the reverse was true, but clicking the symlink in github didn't actually show the content. This way, github renders the docs, and mkdocs can still follow the symlinks in docs/ and generate the docs too.

Test plan

Tested by running

nox -s watch_docs