Docs style guide changes and improvements

Add pointers to civix upgrade instructions

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

Document `civix generate:entity-boilerplate`

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