Add classes to <body> tag of documentation files

To facilitate request from lilypond-user to facilitate differentiating among the various manuals, appending CSS classes to the <body> tag of the documentation files so that the main CSS stylesheet can be modified to look for those classes to specifically style each manual and development status.

The present solution for indicating development status is a hardcoded line in Documentation/lilypond-texi2html.init.
CSS changes to color-code major manuals

Added styling to use a manual-specific class in the <body> tag to color code each of the major manuals.

Now:
* Learning Manual: Green book
* Notation Reference: Blue book
* Usage Manual: Yellow book
* Extending: Red book
* Internals Reference: Purple book
* Contributor's Guide: Black book
Add comments to lilypond-texi2html.init

Adding inline comments to lilypond-texi2html.init to explain the added lines of code in the file to enable the manual-specific CSS styling.

Also cleaned up the code indentation levels.
Remove duplicate CSS definitions in manual file

Remove second definition of background color from lilypond-manuals.css.

Also remove color spec for changes documentation since its color is not being defined at this time.
fix sidebar colors in html documentation

Change the default color of the TOC sidebar when no manual-specific color has been specified. Because the text color was changed to white, the TOC was not legible in manuals like the glossary, which is not assigned a color currently.

Also, darkened most of the manual TOC sidebars to separate from header and footer colors. Lightened the header and footer for the Contributor's Guide since it is the "black book."
Merge branch 'issue3714' into staging