[docs] Doc TOC updates
This PR restructures TOC for the documentation. All the previous links remain working (except the two pages that have been removed: migration guide and converting from TF). 
Here's the scope of the restructure: 
a) TOC is sorted from “beginner” topics to more advanced making it easier to know where an answer to a question might be
b) Some topics have been renamed to be concise with the rest in the same section and (in some cases) more descriptive
c) Task Guides are collapsed by default and now are on on the same level (currently NLP task guides are hidden, and not aligned with other modalities)
d) “General usage” has been  renamed to “Developer Guides”
e) Benchmarks, notebooks, and community resources have been moved under Developer Guides 
f) “Converting from TensorFlow checkpoints” and "Migrating from previous packages" pages removed
_The documentation is not available anymore as the PR was closed or merged._