Asciidoc max include application setting

What does this MR do and why?

This MR should work towards #216108 (closed)

• adds a new application setting asciidoc_max_includes which replaces the old hardcoded MAX_INCLUDES, default value unchanged 32
  • naming is up for discussions
• adds a new general preferences section AsciiDoc which contains a number input for this setting
  • this is debatable and I'm looking forward to input on this
• allows change of application setting via Rails Console

ApplicationSetting.update(asciidoc_max_includes: 5)

• unsure about a sensible max value in the model validation
  • !140531 (diffs)
  • Result: new max value will be set to 64; !140531 (comment 1740507225)

MR acceptance checklist

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Screenshots or screen recordings

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Before	After
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UI changes have been removed per this suggestion.

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. Go to the Settings > General > AsciiDoc
2. Enter a max include setting that fits your test case
3. Add a sample .adoc to your test project and check that the only the numer of configured includes are resolved