Remove warning about development/outdated docs

[nicoddemus]

Unfortunately couldn't figure out how to fix the generated link, so at least
for now remove it to avoid confusion.

While at it, In the 2nd commit I replaced all references from pytest.org/en/latest to pytest.org/en/stable given that now our master contains features for the next release.

Fix #7331

[nicoddemus]

Merging as the failure was just due to timing out to upload to codecov:

curl: (28) Failed to connect to codecov.io port 443: Timed out
    -> Sleeping for 30s and trying again...
codecov-upload.sh: line 1721: [: 0+1: integer expression expected
    X> Failed to upload coverage reports