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".

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/

This makes it easier to refer people to specific guidelines within the
guide.

We really don't need such an exhaustive list of examples in this context.
That material is better handled in the User Guide, which is hyperlinked.
All we need to do is demonstrate the basic idea of a token.