Use `xpath` Nokogiri search for memory & cpu improvements

What does this MR do?

Explores possible improvement of BaseRelativeLinkFilter memory usage.

On my local env, on the example described here (massive note with links), I see a significant improvement:

• 13 GB -> ~30 MB with :optimize_linkable_attributes disabled
• 6.5 GB -> ~30 MB or less with :optimize_linkable_attributes enabled (it prevents for calling the heavy doc search twice)
• It is faster in terms of CPU too on my experiment

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
  • I have included a changelog entry.
  • [-] I have not included a changelog entry because _____.
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

Rely on the existing specs, they are pretty extensive, from what I've seen.
Also, I run these queries manually in rails console on my HTML samples.

Security

N/A

Related to #329186