docs: Generate documentation with Doxygen

doc.wikimedia.org is a helpful resource when iterating on a codebase.
Now that the Desktop Improvements Project is underway, Vector will be
iterated on heavily. Like MobileFrontend, generate documentation with
Doxygen so that it can be published on doc.wikimedia.org.

Note well that:

- The CALL_GRAPH and CALLER_GRAPH statements have been removed as they
  were being set to the default value (NO)

- DOT_FONTSIZE was increased from 10 to 12

Bug: T242779
Change-Id: I7cc2eae2cf950293eee0028873bc6f1c89443977

Doxyfile

@@ -0,0 +1,36 @@
+PROJECT_NAME           = Vector
+PROJECT_BRIEF          = "Vector is a skin for MediaWiki. Vector has been the default skin on all Wikimedia wikis since 2010."
+
+OUTPUT_DIRECTORY       = docs
+HTML_OUTPUT            = php
+
+# Intepret the first line of a comment as a brief description?
+JAVADOC_AUTOBRIEF      = YES
+QT_AUTOBRIEF           = YES
+
+WARN_NO_PARAMDOC       = YES
+
+INPUT                  = .
+EXCLUDE_PATTERNS       = doc docs vendor node_modules tests
+FILE_PATTERNS          = *.php
+
+# Search through subdirectories for input files?
+RECURSIVE              = YES
+
+# NOTE: USE_MDFILE_AS_MAINPAGE requires that the specified file is included as an input file.
+USE_MDFILE_AS_MAINPAGE = ./README.md
+INPUT                 += ./README.md
+
+HTML_DYNAMIC_SECTIONS  = YES
+GENERATE_TREEVIEW      = YES
+TREEVIEW_WIDTH         = 250
+
+GENERATE_LATEX         = NO
+
+HAVE_DOT               = YES
+DOT_FONTNAME           = Helvetica
+DOT_FONTSIZE           = 12
+TEMPLATE_RELATIONS     = YES
+
+# Makes dot run faster. Requires graphviz >1.8.10
+DOT_MULTI_TARGETS      = YES