Book home pages

Conflicts:
	src/AppBundle/Resources/views/Read/book_list.html.twig
	src/AppBundle/Resources/views/Read/home.html.twig

Book categories

Custom theme

Refactor model

Material 1.3.0 was just released with an additional feature to
improve the result when custom-setting the site header to be
non-fixed. This commit implements the necessary changes to take
advantage of this improvement.
Ref: https://github.com/squidfunk/mkdocs-material/issues/205

User Guide - setting master as stable

Above the standard Material header, now there are links that go
to the CiviCRM home page as well as the docs home page

This is the site footer directly from the Material theme,
imported here in prepartion to customize it

Appending "CiviCRM documentation" to the end of all page titles

This is the vanilla htmltitle block used in the Material theme, imported
here in preparation for customizing it.