Add ability to add intro message to MediaWiki:Rules page

[JeroenDeDauw]

Default:

With MediaWiki:Ext-rules-intro containing stuff:

[codecov-commenter]

Codecov Report

❌ Patch coverage is 0% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 59.88%. Comparing base (9eda4d7) to head (03d674a).

Files with missing lines	Patch %	Lines
src/EntryPoints/RulesHooks.php	0.00%	6 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master      #64      +/-   ##
============================================
- Coverage     61.62%   59.88%   -1.75%     
- Complexity       64       65       +1     
============================================
  Files            11       11              
  Lines           172      177       +5     
============================================
  Hits            106      106              
- Misses           66       71       +5     

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