Update-ci

[jgunstone]

This is a back-of-house change to improve the CI.
It replaces mamba with pixi for environment management.

This enables multiple environments, in our case, one to perform the simple validation checks and one to build the documentation.

It also allows for the use of pixi run ... which allows us to template actions in the pixi.toml file to be later used in the GH actions. This means the same templated commands can be used for development and in CI. Check the GH actions files and they are now simpler I think.

Part of the reason for doing this change is also because we will be adding more CI to push the BDNS into BSDD so it will make that simpler.

Other updates:

• added a "dark" theme to the docs
• added a link to Github repo to docs
• added a link to "edit documentation on Github" (requires check valve sprinkler CKVS - Update BDNS_Abbreviations_Register.csv #337)

I've checked it on my own fork and am confident it works.
Unfortunately I can't make this mornings meetings but if people are happy to merge then just add the review and then I can do the merge after the meeting. (Otherwise it can wait til next week)

Cheers