Fix missing reference to pre/postJob hook files.

add more informatoin about creating an issue

CRM-21460 Add job execution hooks

Update setting metadata information

Remove reference to adding `CRM_Contact_BAO_Query::getGroupByFromSele…

dev/core#90 Add in documentation of the disabling and re-enabling of …

Update hook_civicrm_export

Describe new variables due to dev/core#457

Adding this clause has led to unreliable query results - better to give no information than to lead them wrong - many reports now disable & reenable full group by but that is referred to elsewhere.

Also better to override whereClause & selectClause

add 'CIVICRM_TEMP_FORCE_DURABLE'

Note working through this threw up some ambiguity about help_text vs help which I'll dig into as a follow up. I think there is further text on the page I need to review too about the settings form

dev/core#90 Add in documentation of the disabling and re-enabling of the ONLY_FULL_GROUP_BY sql mode for developers to use

Add in documentation of new hook hook_civicrm_postIPNProcess

Clarifications to buildkit path docs

Fix typo in hook_civicrm_links 

Remove unnecessary "are" in upgrade.md

Continuous Integration - Improve layout and add section for extension PRs

Cache Reference - Link to admin guide. Add section on custom caches.

Update hook_civicrm_pre.md

Update hook_civicrm_post.md

Update debugging.md

Fix comma splice.  Make clearer reference to later section.

* Emphasize use-cases first -- and then drill-down to tools/specifics.
* Add a placeholder section for extension PRs
* Re-style and copy-edit the section for core PRs

Update info on civix generate:api