Update Doc For New API Action 'getunique'

Embed video walk-through for PR review

ref: https://jameshfisher.com/2017/08/30/how-do-i-make-a-full-width-iframe.html

Add docs for hook_civicrm_alterLocationMergeData

Create interacting-with-core

GitLab for issue tracking

Also:

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

trivial: remove layer of quotes

Resolve #491 Reference hook_civicrm_coreResourceList in the advanced …

Docs style guide changes and improvements

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.

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.