Also:

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

Two arrows were being interpreted as two layers of quotes. I removed one of them. Then I thought, actually, these are not quotes. Maybe we should remove move. And maybe just having them on their on paragraph without any quotes is good enough. I am shying away from using boxes for this since to avoid box fatigue but would be interested in some opinions on the house style.

We ha one "Generating a skeletal extension" and one "Add features"

Add health warning about running Docker on OSX

Per https://en.wikipedia.org/wiki/Uniform_Resource_Identifier#URI_references
it seems that something like `/foo/bar.md` isn't technically a "URL".
So to be more accurate with our language, this commit changes the meta
docs to refer to internal "links" instead.

This is a significant revision of the PHPUnit documentation. This specifically:

* Adds concrete examples of writing tests in various styles (`CiviUnitTestCase`, `CiviEndToEndTestCase`, `civix`)
* Adds reference documentation for classes/traits/interfaces provided by Civi's test library
* Imports key information from Testapalooza docs (#389)

Changed `civix generate:custom-data` to `civix generate:custom-xml` as the former is incorrect. Also corrected grammar in subheading "Extending a subtype".

Resolve #491 Reference hook_civicrm_coreResourceList in the advanced options in Resource framework reference

There are usually a few branches to choose from when picking a base-branch.
This breaks out a table to differentiate them.

Include Survey in the list of pre and post entities and alpha sort the post entities like the pre entities

Use the terminology "Chapter/Sub-chapter/Page/Section" instead of
"Part/??/Chapter/Section" because it's more intuitive.

Some places in the Dev and Sysadmin guides have deviated from this
guideline and I think it's for the better.

Examples:

* https://docs.civicrm.org/dev/en/latest/framework/angular/
* https://docs.civicrm.org/sysadmin/en/latest/integration/drupal/views/