Linting: create check for headings which start with a number

[ScatteredComet]

• Adds a linting check using python to find any headings which start with a number and do not have a preceding custom anchor. If there is no preceding anchor, the link fragment will appear incorrect as per Incorrect link fragments #11016

This is a draft PR for a few reasons:

• This check will fail until Incorrect link fragments #11016 is fixed e.g. by merging Fix section heading link fragments which start with a digit #11811
• I don't know if enforcing this is allowed; maybe its a bad idea
• If this lint is allowed, it should probably be described in the contributing docs e.g. here and that section should be referenced by the error message instead