.circleci: Remove migrated jobs, move docs builds (#64222)

.circleci: Remove migrated jobs, move docs builds (#64222)

Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/64222

Removes both backwards_compat as well as docs_test from the general
gcc5.4 config and moves the docs build from being run on every PR to
only being run on master.

We can remove docs builds when we migrate the docs push job (including
all secrets associated with that)

Signed-off-by: Eli Uriegas <eliuriegas@fb.com>
cc ezyang seemethere malfet walterddr lg20987 pytorch/pytorch-dev-infra

Test Plan: Imported from OSS

Reviewed By: malfet

Differential Revision: D30650953

Pulled By: seemethere

fbshipit-source-id: ac11da6a551a6c81f3dc1d47fd81846cbfe9975a