Deploy doc rendering for v2.1

[andreasabel]

From https://agda.zulipchat.com/#narrow/stream/259646-announce/topic/Standard.20library.20v2.2E1/near/455502043

When will https://agda.github.io/agda-stdlib/v2.1/ be available?

NB: Listing the docs under https://agda.github.io/agda-stdlib will probably remain a manual process, but pushing the docs to subfolder tag for a new tag tag could probably be done by CI.

[jamesmckinna]

@gallais is this still a manual process, or already part of the CI workflow?

[gallais]

Manual (but documented in the release guide)