add FixtureManager to docs

[svenevs]

Fixes #3420.

@nicoddemus encouraged me to submit a PR, but please understand I have not been able to render the docs locally to verify that this documentation links / finds the class correctly (error in third party docs extension).

Commits from maintainers can be done on that fork, as well as I'm happy to move it to another location on the docs etc. Thanks for pytest, the learning curve was totally worth it ❤️

[coveralls]

Coverage increased (+0.05%) to 92.856% when pulling f5a19bc on svenevs:master into 0b2b73c on pytest-dev:master.

[nicoddemus]

Thanks a lot for following up with the PR @svenevs, we appreciate it!

Glad to know you are enjoying working/learning pytest! 👍

[nicoddemus]

Uh oh, sorry to only realize this now: the FixtureManager is not really part of the public API, so it should not go to the docs. The reference docs is meant only for public API the user calls directly or is accessible from public objects, but the FixtureManager is an implementation detail that we reserve the right to change in the future as part of an internal refactoring or implementation of a new feature.

I'm really sorry I answered to your question in #3420 (comment) in haste and made you waste your time preparing this PR. 😞

Again thanks for the PR, totally my fault for suggesting to write a PR, but please do stick around to work on something else if you feel like. Thanks for understanding!

[svenevs]

No problem, creation of the PR took maybe 5 minutes since I saved the diff 😄

[nicoddemus]

Hehehe great, thanks!