Update docs-build job (#1012)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/1012

- Add logic for all html pages in main to be excluded from Google Search.
- Fix the sed script to strip first 'v' in the tag name, as well as patch version and rc. So that 'v0.1.2' -> '0.1' and 'v0.1.2-rc1' -> 0.1.

Reviewed By: cccclai

Differential Revision: D50342373

fbshipit-source-id: 97acc8420bcc410354906510385f08c871495967