docs: Convert doxygen_first_page.php to proper Markdown

docs: Convert doxygen_first_page.php to proper Markdown

Doxygen has native support for indexing of Markdown pages.
It basically parses them the same way it parses the uglier
methodology of creating a PHP file containing a the markdown
text in a large comment that would start with `@page` or `@mainpage`.

* http://doxygen.nl/manual/commands.html#cmdpage
* http://doxygen.nl/manual/commands.html#cmdmainpage
* http://doxygen.nl/manual/markdown.html#md_page_header

The internal reference name and page title (comparable to the string
passed to `@mainpage` or `@page`) are by default derived from the
file name (without suffix).

The internal reference name (for use in cross-page links) can also
be overidden with an `{#something}` annotation in the first heading.

When using the annotation `{#mainpage}` it changes the page type
from a regular `@page` to the `@mainpage`.

Change-Id: I0c25a49088825744773c4d87dde5688894b2004e