Baseline copyediting and correctness pass to update existing doc content

[CAM-Gerlach]

Issue Description

Currently, quite a few details and references in our documentation are out of date (e.g. to Mercurial support, Python(x, y), the non-IPython console, etc), and a number of grammar, style, and syntax problems exist. In this issue, I propose a baseline sweep through the existing stuff to clean up such easy to resolve items to ensure a baseline of textual quality for our initial public release of the new docs.

Task List

• Decide on conventions and create internal style guide (public version to be released as a separate issue/PR)
• Initial cross-file changes
  • Use consistent capitalization, marker characters, naming scheme, and structure for titles, headings and subheads, and add additional
  • Rename "Related Plugins" section to "Related Components", make a L3 subhead, and cleanup/add additional links
  • Harmonize all topic names to be bold and title case in first sentence
• Index page
  • Replace outdated content with heavily revised text from readme
  • Make one sentance per line
  • Re-organize and cleanup toctree
• Baseline copyediting and corrections per file
  • Remove/correct outdated information
  • Copyedit text for better prose
  • Harmonize line breaks to one per sentence to make both editing and reviewing changes much easier, and git much cleaner and more effective
  • Add minimal necessary additional text/items to fill out at least one short section per doc
• Add a few additional brief descriptions to bare links in overview
• Fix numerous bad links and add https where possible
• Harmonize formatting of elements/directives to conform to style guide
  • Admonitions
  • Set two blank lines before all headings and overbar format for L2
  • File names/paths
  • Keyboard shortcuts
  • GUI elements
  • Menu/preference selections
  • Code blocks