Previously this was necessary to get syntax highlighting to work, but
now just doing ```php will work.

Move them into the dedicated page on PhpStorm. Why? Tim has expressed
some concern about the potential difficulty (and slippery slope) of
starting to maintain IDE-specific docs. I like the PhpStorm page, but I
can see where he's coming from to a certain extent. As such, I think it
will be a good compromise if we can keep all the IDE-specific stuff as
consolidated as possible.

ref:
https://docs.civicrm.org/dev/en/latest/documentation/style-guide/#parts-chapters-sections

As noted here https://github.com/civicrm/civicrm-buildkit/issues/333
I'm trying to move us away from using that dedicated site.

Remove "additional pages" section from AngularJS intro page

because these pages are already listed in the menu and I think that
listing them in two places will be prone to content errors

CRM-20600 - Add chapters for AngularJS and AssetBuilder

Minor format changes: hook_civicrm_batchItems.md

Made minor formatting changes to improve appearance of rendered file.

Update extensions.md    "book(s)" to "guide(s)"

close #131