[End User Docs] Spec

We should have really nice end user documentation.

### Profiler - how it works
* [x] Probabilistic samples
* [x] Deterministic markers

### Header
* [ ] What are threads
* [ ] Main/content threads
* [ ] ThreadStack graph
* [ ] Markers
* [ ] Thread responsiveness

### Workflows
* [ ] Profile sharing
* [ ] How to file a perf bug
* [ ] Gecko profiler add-on
* [ ] Mobile profiling

### Call tree
* [x] Summarizes information
* [x] How it is generated
* [x] What is a Call Node?
* [x] Inverting
* [x] Filtering implementation
* [ ] Call tree transforms
* [x] Searching
* [ ] Workflows

## Stack chart
* [ ] Lays out information over time.

### Marker chart
* [ ] Explain markers
* [ ] Known markers documentation
* [ ] How to add markers
* [ ] UserTimings

### Other types of documentation
* [ ] Document workflows
* [x] Integrate [video documentation](https://www.youtube.com/watch?v=MxgWOTqxOTg&list=PLxaZqnd-OQM620EZ_6eT8qurOnZ4eu6dz)

### Understanding performance
* [ ] Provide recommended reading links
* [ ] Big O and complexity
https://www.desmos.com/calculator/dlh7cpeav1
https://rob-bell.net/2009/06/a-beginners-guide-to-big-o-notation/
* [ ] Optimize only things that are slow
* [ ] How to find slow code
* [ ] How to fix slow code
* [ ] JS: Fixing GC issues
* [ ] C++: Code locality
* [ ] Lazy loading, only compute what is shown
* [ ] Caching: Don't re-compute

┆Issue is synchronized with this [Jira Task](https://mozilla-hub.atlassian.net/browse/FP-703)