non-deprecated hooks

migration testing

custom header IDs

Feature/update planning

Various spelling, grammar, and clarity of meaning improvements.

Corrected the incorrectly used word 'thought' to 'through'. 
Corrected grammar of 'how to write an extensions'. 
Clarified the meaning of 'ensuring that you really need to start coding'.
Added full stops to 'aka' abbreviation to make it clearer it is an abbreviation.
Changed 'developer environment' to more conventional and accurate phrase 'development environment'.

style guide - calling the "User Guide" the "User Guide" instead of the "User and Administrator Guide"

update from core

* documenting custom header IDs
* removing rule about using one H1 element (that's in style guide now)
* clarifying thad admonitions only work when enabled

https://pythonhosted.org/Markdown/extensions/attr_list.html

Civix - some small clean up

markdown rules - specifying that internal hyperlinks should use absolute paths with preceding slashes

EXT-21: Migrated/updated documentation around extension publication and automated distribution