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 

Changed "book(s)" to "guide(s)" throughout.

NOTE: No change: the two following lines have links with "book(s)" embedded in them.
Line no.	issue
12		[/**books**/volunteer.yml](https://github.com/civicrm/civicrm-docs/blob/master/**books**/volunteer.yml)
101		Go to the [**books** configuration within our publishing system](https://github.com/civicrm/civicrm-docs/tree/master/**books**)

Update index.md changing "book(s)" to "guide(s)"

Close #131 

Changes include "guide book(s)" to "guide(s)" in par. 3 and 4 (lines 8, 10).

Migrated from the following two sources

https://github.com/civicrm/civicrm-buildkit/blob/d20229ac9f63aaf183687157a5e655c953b81f5e/doc/daily-coding.md#housekeeping
https://civicrm.stackexchange.com/questions/17717/how-do-i-upgrade-civicrm-on-a-local-site-that-i-installed-with-buildkit-civibuil/17721#17721

Not adding internal redirects because these pages are so new.

New sections on dev tools and buildkit

Based on discussion here:
https://github.com/civicrm/civicrm-dev-docs/pull/189

Content in it needs to be re-oragnized into multiple other pages. We'll
leave that for another time.

Update index.md, paragraph 2 -- "basic overview" link

In second paragraph remove space between "overview] (https" to read "overview](https:" so that URL does not appear adjacent to "basic overview".