docgen: add source URLs to the default formatter

[oandregal]

As of #14656 the generated docs no longer include the URL to the export statement.

These are some problems we faced:

• Relative URLs work well for github or a local git repo, but they doesn't for npm or the handbook.
• Using the <> symbol to point to code had a weird formatting in the handbook due to escaping (see Remove/docgen url symbol #14656).
• We may want to consider pointing to the symbol definition instead of/besides pointing to the export statement. See Testing: Reimplement pressKeyWithModifier to emulate Cmd+A in macOS #14243 (comment)

[aduth]

If I'm being honest, I don't know that I long for the URL to return. I think it could be argued that it provides some value, but also a fair amount of pain in the line-number-only revisions it has a tendency to introduce to a README file in the course of a pull request.

[oandregal]

I share that sentiment. One thing that is growing on me is that its usefulness depends on context:

• I'm all in for having the URL in the handbook. That's something that you navigate with the browser, and you may want to have a quick look at the implementation.

• README files that are collocated with code: it's nice, but doesn't provide the same value.

Right now, both use the same README. But there is no reason they should (specially if/when we consider moving the symbols to developer.wordpress.org).

I don't feel the urge to prioritize this.

[oandregal]

I'm triagging issues we don't plan to tackle. We can reopen if it regains interest.