OWD project: Update MDN Performance API docs

[mmocny]

A small group of engineers spent about 30 minutes reviewing existing Performance API and Performance Timeline documentation to search for any opportunities for improvement.

We found many examples of improvement opportunities:

For example:

• Lack of a general "Performance Guide"
  • Perhaps can borrow from the explainer with some updates?
  • Perf APIs primer should perhaps be turned into an MDN page.
• Issues in references pages:
  • e.g., In PerformanceObserver reference the description for observe() method does not match the descriptions from the actual observe() method reference page. Both descriptions could use updates.
  • e.g., LCP, ElementTiming, and others do not describe the inherited attributes from PerformanceEntry even though their semantic meaning is specific to the API in question.
  • e.g. PerformanceLongTaskTiming reference is basically empty.
• Issues with Examples throughout reference pages.
  • e.g., dictionary options passed to PerformanceObserver.observe() use entryTypes instead of type and buffered.
• Certain APIs are labeled experimental when they should no longer be (example PerformanceObserver)
• Some new experimental apis are not documented, while some long abandoned apis are still documented (e.g. Frame Timing).
• Various "see also" links are out of date (example)

We think improvements in this area would be very valuable, and greatly appreciated!

[mmocny]

I'll add, several Web Performance WG members have already expressed interest in helping further (for e.g. reviewing pull requests, or consulting on content).