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

docs/Introduction.md

@@ -0,0 +1,7 @@
+Introduction {#mainpage}
+=======
+
+Welcome on MediaWiki autogenerated documentation system.
+
+If you are looking to use, install or configure your wiki, you probably
+want to look at the main site: <https://www.mediawiki.org/>.

docs/doxygen_first_page.php

@@ -1,19 +0,0 @@
-<?php
-die( "Not a valid entry point\n" );
-/**
- * This file does not hold any code. It is only there so we can generate
- * the doxygen documentation main page.
- *
- * @file
- */
-
-/**
- * @mainpage Introduction
- *
- * Welcome on MediaWiki autogenerated documentation system.
- *
- * If you are looking to use, install or configure your wiki, you probably
- * want to look at the main site: https://www.mediawiki.org/
- *
- * @note this page is generated from docs/doxygen_first_page.php
- */

maintenance/Doxyfile

@@ -145,6 +145,7 @@ WARN_LOGFILE           =
 INPUT                  = {{INPUT}}
 INPUT_ENCODING         = UTF-8
 FILE_PATTERNS          = *.php \
+                         *.md \
                          *.inc
 RECURSIVE              = YES
 EXCLUDE                = {{EXCLUDE}}