Update front-page docs to point to the new forum category (#1051)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/1051

The forums don't use hashtags, they use categories. We now have the `ExecuTorch` category, so point to it.

Also, while I'm modifying the main docs page, explicitly point to the github repo to address T167412145.

Reviewed By: mergennachin

Differential Revision: D50510494

fbshipit-source-id: c18bd73ac4bcfbea277fd19868de56ea3e5bfc94