archwiki: update robots.txt

- The /index.php? rule disallows all URLs with this prefix, so using
  longer prefixes does not make sense.
- Allow indexing of the Help: namespace which contains relevant text.
- Keep the intention of disallowing special namespaces, including the
  File: alias for Image: and using the /title/ path and colon separator
  for the namespace prefix.

roles/archwiki/files/robots.txt

 User-agent: *
 Disallow: /index.php?
-Disallow: /index.php?diff=
-Disallow: /index.php?oldid=
-Disallow: /index.php?curid=
-Disallow: /index.php?title=Help
-Disallow: /index.php?title=Image
-Disallow: /index.php?title=MediaWiki
-Disallow: /index.php?title=Special:
-Disallow: /index.php?title=Template
 Disallow: /skins/
+Disallow: /title/File:
+Disallow: /title/Image:
+Disallow: /title/MediaWiki:
+Disallow: /title/Special:
+Disallow: /title/Template: