ci: don't install mkdocs material insiders

[AstreaTSS]

Pull Request Type

• Feature addition
• Bugfix
• Documentation update
• Code refactor
• Tests improvement
• CI/CD pipeline enhancement
• Other: [Replace with a description]

Description

For a while now, the docs CI hasn't worked because it's trying to use mkdocs-material-insider, which we no longer have access to. We're not using anything from Insider that is critical (it's mostly the privacy plugin and a few tweaks to the looks), so this PR removes it altogether from our CI.

Changes

• Remove installing Insider from the CI.

Related Issues

Test Scenarios

Python Compatibility

• I've ensured my code works on Python 3.10.x
• I've ensured my code works on Python 3.11.x

Checklist

• I've run the pre-commit code linter over all edited files
• I've tested my changes on supported Python versions
• I've added tests for my code, if applicable
• I've updated / added documentation, where applicable