docs: Generate documentation with Doxygen

doc.wikimedia.org is a helpful resource when iterating on a codebase.
Like MobileFrontend, generate documentation with Doxygen so that it can
be published there.

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: I41e4c6ca2b604382b036606ebcb40f11a1e8abc0

.gitignore

@@ -1,5 +1,6 @@
 /docs/js/
 /docs/ui/
+/docs/php/
 /.nyc_output/
 /node_modules/
 /vendor/

Doxyfile

@@ -0,0 +1,36 @@
+PROJECT_NAME           = Popups
+PROJECT_BRIEF          = "Displays page/reference previews when hovering over a link to an article/reference."
+
+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