Add and use a separate yml for building docs

[schallerdavid]

Description

This PR adds a separate yaml for installing sphinx dependencies.

Todos

Notable points that this PR has either accomplished or will accomplish.

• [x] add docs_env.yaml
• [x] remove sphinx dependencies from test_env.yaml
• [x] specify new docs_env.yaml in docs building workflow
• [x] all tests pass

Status

• [x] Ready to go

[codecov-commenter]

Codecov Report

Merging #104 (d6982b4) into master (93d9cd4) will increase coverage by 0.39%. The diff coverage is n/a.

[schallerdavid]

I will merge this PR without review since there is likely no harm to the KinoML framework.