improve documentation formatting and pandoc compatibility

look at this shit and tell me you see no problem:
https://repology.org/metapackage/pandoc/versions

doc/meson.build

@@ -6,14 +6,52 @@ if get_option('docs')
         error('pandoc is required to build documentation. Please install it, or disable the `docs` option if you don\'t want it.')
     endif
 
+    r = run_command(pandoc, '--list-input-formats')
+
+    if r.returncode() != 0
+        error('Couldn\'t retrieve list of input formats supported by pandoc (error code @0@)\n\n@1@'.format(
+            r.returncode(),
+            r.stderr()
+        ))
+    endif
+
+    formats = r.stdout().split()
+    input_format = 'ghc'
+    input_format_fallback = 'markdown_github'
+
+    if not formats.contains(input_format)
+        message('pandoc doesn\'t support the \'@0@\' format, falling back to @1@'.format(
+            input_format,
+            input_format_fallback
+        ))
+
+        if not formats.contains(input_format_fallback)
+            error('pandoc supports neither \'@0@\' nor \'@1@\''.format(
+                input_format,
+                input_format_fallback
+            ))
+        endif
+
+        input_format = input_format_fallback
+    endif
+
     md_to_html = generator(pandoc,
-        arguments: ['--from', 'markdown_github', '--to', 'html', '--standalone', '@INPUT@', '--output', '@OUTPUT@'],
+        arguments: [
+            '--from', input_format,
+            '--to', 'html5',
+            '--standalone',
+            '--self-contained',
+            '--table-of-contents',
+            '@EXTRA_ARGS@',
+            '@INPUT@',
+            '--output', '@OUTPUT@',
+        ],
         output: '@BASENAME@.html',
     )
 
     md_docs = [
-        'ENVIRON.md',
-        'GAME.md',
+        ['ENVIRON.md', 'Environment Variables'],
+        ['GAME.md', 'Game Manual'],
     ]
 
     txt_docs = [
@@ -33,11 +71,16 @@ if get_option('docs')
         )
     endforeach
 
-    foreach f : md_docs
+    foreach pair : md_docs
+        f = pair[0]
+        title = pair[1]
+
         name = f.split('.')[0] + '.html'
         custom_target(name,
             command : [eolconv, eol, '@INPUT@', '@OUTPUT@'],
-            input : md_to_html.process(f),
+            input : md_to_html.process(f, extra_args : [
+                '--metadata=title=@0@ - Taisei Project'.format(title)
+            ]),
             output : name,
             install : true,
             install_dir : doc_path