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.

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

These documents are misleading -- they talk about CiviMail because, *historically*,
much of this grew out of CiviMail. However, *today*, tokens are used more generally.
Also, in `civicrm-core`, `TokenProcessor` is *not* used by CiviMail. (You have to
install FlexMailer to use it.)

The old section signal a few things:

1. "CiviMail design needs to handle HTML/Text" - but this can be more concisely stated as part of he requirements summary. Added a blurb there.
2. "The `{action.unsubscribe}` is used differently in each context" - but this is better explained as part of the *User Guide* for CiviMail.

Arguably, in the context of CiviMail developer docs, we should also signal "How to manage token content with text-vs-html", but this section
doesn't actually do that, and that's already signalled in the examples of extending the token system.