portdocs to fslab theme

[kMutagene]

Proposed Changes

This PR ports the existing docs to the fslab documentation theme

Types of changes

What types of changes does your code introduce to RProvider? Put an x in the boxes that apply

• [ ] Bugfix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [x] These changes only affect the documentation

This PR

• Updates the used fsdocs version
• disables the execution of all graphics functions such as R.plot in the documentation scripts (meaning that no popup windows will happen on doc generation - the code will still be included in the docs pages though!)
• Uses the fslab documentation theme for all docs pages

Checklist

Put an x in the boxes that apply. You can also fill these out after creating the PR. If you're unsure about any of them, don't hesitate to ask. We're here to help! This is simply a reminder of what we are going to look for before merging your code.

• [x] Build and tests pass locally
• [ ] I have added tests that prove my fix is effective or that my feature works (if appropriate)
• [ ] I have added necessary documentation (if appropriate)

[kMutagene]

@AndrewIOM Seems like something in the CI overwrites the custom.css file (the one on gh pages is empty). Quite strange as fsdocs watch --eval works flawlessly local. ill take a look at the build target

[kMutagene]

@AndrewIOM nvm seemed to be a cache issue. looks as expected now