Roll back to older versions of Sphinx

[kipiberbatov]

Fix the Sphinx 9.1.0 error in https://github.com/neperfepx/neper/actions/runs/25075778039 by rolling back to versions prior to Sphinx 8.

[kipiberbatov]

@rquey Great, now commits to main trigger sucessful (to some extent) run of the workflow jobs!
There are few more issues that remain:

1. Remove the GitHub's Node.js warnings (fixed by Update versions of official GitHub Actions kipiberbatov/neper#5).
2. Fail gracefully when deployment happens via a branch.
   On my fork I have enabled GitHub Actions and so I successfully build a minimal version of the website: https://kipiberbatov.github.io/neper/.
   On the other hand, here (in the upstream) deployment happens via a branch, and so an error occurs.
   Indeed, in https://github.com/neperfepx/neper/blob/ec2ce57877d9292421a41c2fcdcb9365a72c3d28/.github/scripts/check-pages-enabled.sh I only check whether GitHub Pages is enabled but not how does deployment happens (which makes the deploy workflow to crash: https://github.com/neperfepx/neper/actions/runs/25100534259/job/73548389494).
   I am researching how to fix that.
3. Fix the tests.
   Some of the tests fail, and there are differences in the 4 cases ({linux,macos}-{arm,x86_64}), and this will need a further examination.
   For this reason there are continue-on-error: true rules in https://github.com/neperfepx/neper/blob/ec2ce57877d9292421a41c2fcdcb9365a72c3d28/.github/workflows/linux.yaml and https://github.com/neperfepx/neper/blob/ec2ce57877d9292421a41c2fcdcb9365a72c3d28/.github/workflows/macos.yaml, so that the workflows execute untill the end (but this should be considered only a temporary hack).