Document the flamegraph features in Perfetto

[LalitMaganti]

We've done a lot here and it's worth documenting this.

We might want to wait until if/when tree explorer lands and document it all as a unit.

[SankeerthNara]

Could you assign this to me

[LalitMaganti]

I'd like the core team to work on this issue please. I'd like to wait a bit until some upcoming work lands as well.