Docs: Quote <!--more--> in <code> instead of italicizing it

Also replace "--" with "&#x2d;&#x2d;" prevent "--" from
being interpreted as en-dash.

Note: A simple `<!--more-->` does not work.
Docs: Minor copyediting/proofreading

- Correct some typos
- Add backticks and commas where necessary
- Use fenced code blocks specifying "bash" as the language
  to avoid weird highlighting
- Place commas outside of quotation marks surroundingn codes
  to avoid possible confusion
- Suggest users to use the discussion forum rather than the
  mailing list
Fix front matter for 'Creating a New Theme'

There seems to be an issue with TOML and certain menu structures. Current workaround: YAML.