Installing HTML documentation

[comburent]

Feature request:
Having the HTML docs installed for a better self-contained/hosted app.

Suggestion:

• Generate and install the HTML documentation of https://jcorporation.github.io/myMPD/ to a path specified using a new -Dhtmldir=… cmake option. (defaults to /usr/share/doc/mympd/html/… if not specified)
  This step could be made optional through MYMPD_HTMLDOC for distributions not interested in it.

[jcorporation]

The HTML documentation is created by jekyll and the API documentation parts by doxygen and jsdoc. Therefor installing the html documentation locally requires many big dependencies.

[jcorporation]

Next version of myMPD will install as default the CHANGELOG.md, LICENSE.MD, README.md and SECURITY.md files. There is now also an option create and install the html documentation.

OPTION	DEFAULT	DESCRIPTION
MYMPD_DOC	ON	Installs documentation
MYMPD_DOC_HTML	OFF	Creates and installs the html documentation