Warn when a heading contains an inline applies_to

[colleenmcginnis]

Warns when a heading contains an inline applies_to.

@theletterf and I paired today on adding a warning when we detect inline applies_to syntax in the heading text. We tried a couple different approaches. This one works, but I'm not sure if it's the best place to check for this.

Open questions for @elastic/docs-engineering:

• Is this an appropriate place to check for applies_to syntax in a heading?
• Does it make sense to use regex to check?
• Is there a way to add more context (like the line number and highlighting the text that's causing the error) to the warning message? Right now it just looks like this:
  

[theletterf]

I find this solution pretty clean, given that inline roles can potentially be placed anywhere!

[cotti]

My 2c: The placement looks right!

In order to get the extra info (and clean up the call to EmitWarning), this branch needs to extend ProcessorDiagnosticExtensions to have at least an EmitWarning and its accompanying Emit for BlockProcessor.

After it's implemented, it can be called just with a processor.EmitWarning(block, "Do not use inline 'applies_to' annotations with headings. Use a section 'applies_to' annotation instead.");

[Mpdreamz]

I just implemented it without looking at @cotti's review but ended up doing the same thing. NICE!