Add link page on PR reviewing

This way MkDocs will be able to report errors for broken links

Done with:

perl -0777 -i -pe \
's@(?<!!)\[([^\]]*)\]\((?!https?://)(?!mailto:)(/(?:(?:(?:/(?![\)#]))|[^/)#])(?!.md))+)/?(#[^)]*)?\)@[$1]($2.md$3)@gm' \
`find . -name '*.md'`

Add links to other documentation, clean up grammar and formatting, clarify a couple points

since these list items are all also headings and thus will appear in the
TOC nav on the right side of the page

per comments from @eileen here https://github.com/civicrm/api4/issues/14#issuecomment-278860730

https://wiki.civicrm.org/confluence/display/CRMDOC/Database+Reference