Say "internal link" instead of "internal URL"

Update buildkit.md

Add health warning about running Docker on OSX

Tokens/Mailings - Split in two files. Multiple cleanups.

tests/phpunit.md - Document multiple helper classes. Provide specific examples of writing tests.

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)

Corrected civix generate:custom-xml

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

Remove bold from headers

Update the list of Pre and Post EntityTypes after doing…

Example link for searlization change

Add to change log re any entity custom field & serialisation metadata

toosl/git.md - Describe base branches in more detail

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

Add serialize to schema

Refer to example of good way to add pre/post hooks to core

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

Update style-guide.md

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.

tools/jenkins.md - Add tips for reviewing console output