use sphinxmark to add draft watermark to all pages

[jstriebel]

This is a POC for a draft watermark using the sphinxmark extension. Currently it's only possible to mark all pages or none, see kallimachos/sphinxmark#5. Might fix #170.

[joshmoore]

https://zarr-specs--180.org.readthedocs.build/en/180/extensions/data-types/complex/v1.0.html

That's exceedingly clear. Open to opinions on if it makes reading difficult.

[jstriebel]

@joshmoore I saw that #168 also accidentally updated the theme. There were some nice additions, but some things also felt off (e.g. the content was too narrow on large screens IMO). You can see version after the update https://zarr-specs.readthedocs.io/en/latest/core/v3.0.html, and the version with the changes from this branch here. If the updated theme does not fit, we could also pin the previous version, which was 0.8.0.

I also used a more light-weight draft watermark now:

What do you think about the changes @joshmoore @MSanKeys963?

[sanketverma1704]

Hi @jstriebel. Thanks for sending the PR.

I had a look at the current version and one in this PR. I prefer the one in this PR here. This one is less narrow and better utilises the spaces on the left and right-hand side of the screen. This may be a personal preference, and I'm happy to see what others think.