Top level documentation navigation revamp

[mergennachin]

Tracking Issue for discussion #8178

cc @byjlw

[byjlw]

Big fan of tracking items, let's just make sure there is enough clarity in the issue for someone to know what problem it solves and what success looks like

[mergennachin]

Navigation proposal: https://docs.google.com/document/d/1qxo3Tkaz5QPxPyH1ckDoVwOEfjzPU5BwHKSLEt6PIcg/edit?tab=t.0#heading=h.1q2j4q4smd2b

[GregoryComer]

I'm planning to polish off the remaining few empty doc pages and cross-links on the executorch-just-works branch, then try to merge back into master by the 20th (I believe this was the original goal). The existing mobile and backend docs can serve as placeholders, and there's a lot of OOTOs this week. I'm hoping to get back onto master, as it will be much easier to distribute docs work there.

From there, I'm hoping to recruit some help to work through the backends and platform docs. iOS docs have pretty good coverage currently - I've bumped them up to the top level to be more obvious to new users. I could likely use @kirklandsign's help to populate the Android docs once he's back. For backends, I'm hoping to get the XNNPACK backend docs into a good state where we can use it as a model or reference for the others. From there, I'm intending to find volunteers to help fill out the remaining docs.

CC @mergennachin @byjlw

[GregoryComer]

See #8676.