Update release process

[benjaoming]

Proposal to:

1. Update the release process to always release from master
2. Add a section "verification" of steps that can be done to verify a local package
3. Dockerize the entire release process to have containerized and "reproducible" environments -- this means to address some of the proposed requirements in Revisit Docker configuration pattern #1337 to also include the release preparation

[benjaoming]

Also, this issue is entirely because I made a couple of mistakes in the pre-release process for 1.1.0 and I can hash out steps to ensure that doesn't happen again 😇

[humitos]

We have this process documented at https://sphinx-rtd-theme.readthedocs.io/en/stable/contributing.html#releasing-the-theme