Fix CI Builds

[SamJUK]

Fixes the failing CI builds, that was caused by loose dependencies in the upstream actions. Ref: #24 (comment)

(Happy to split into multiple smaller PR's if preferred).

Example CI Runs & Resulting build

On PR: https://github.com/SamJUK/warden-docs/actions/runs/11882793013
On Main push: https://github.com/SamJUK/warden-docs/actions/runs/11882799248
Resulting documentation: https://samjuk.github.io/warden-docs/

Changes

• Pins the python package versions, hopefully should improve reproducibility of builds.
• Update Sphinx to the latest version (8.1.3), and remaining packages to latest compatible versions.
• Removes redundant configuration, that triggers warnings in the build process on the new Sphinx version.
• Bumps Github Actions versions to improve performance & remove deprecation warnings from the build logs.