Co-Authored-By: Mikey O'Toole <mikey@mjco.uk>

show new way for accessing pr test site credentials

ref: https://jameshfisher.com/2017/08/30/how-do-i-make-a-full-width-iframe.html

Also:

- Clarify that PRs don't need to have associated tickets
- Clean up some language here and there

Remove "editorial" TODO item from "If the PR contains database upgrades".

Reading for content; editing for consistency.

Reading for content; editing for consistency.

Reading for content; editing for consistency. 
Note:  Check for Drupal dependencies in the DB upgrade case.

Reading for content; editing for consistency.

Revised edit to link to dev-docs/basics material for talking with CiviCRM developer community.  Thanks, Sean.

The example command works if you've manually installed the `hub` with all
the suggestions in upstream's guide -- specifically, they suggest aliasing
`git`==`hub`.  However, in `civi-download-tools`, we download the `hub`
command but can't setup the alias.

Documenting commands as `hub <foo>` should make them more robust/portable to
more configurations.

https://wiki.civicrm.org/confluence/display/CRMDOC/Verify+a+bug+fix

https://github.com/civicrm/civicrm-core/blob/master/.github/CONTRIBUTING.md

Reorder requirements docs into the basics section and move devloping civicrm into core and add backdrop in

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