docs: Add docs for thermoelastic3d

[g-braeunlich]

Description

This adds docs for thermoelastic3d, which I missed in the previous PR

Type of change

Please delete options that are not relevant.

• Documentation only change (no code changed)
• Bug fix (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)
• This change requires a documentation update

Checklist:

• I have run the pre-commit checks with pre-commit run --all-files
• I have run ruff check . and ruff format
• I have run mypy .
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

Reviewer Checklist:

• The content of this PR brings value to the community. It is not too specific to a particular use case.
• The tests and checks pass (linting, formatting, type checking). For a new problem, double check the github actions workflow to ensure the problem is being tested.
• The documentation is updated.
• The code is understandable and commented. No large code blocks are left unexplained, no huge file. Can I read and understand the code easily?
• There is no merge conflict.
• The changes are not breaking the existing results (datasets, training curves, etc.). If they do, is there a good reason for it? And is the associated problem version bumped?
• For a new problem, has the dataset been generated with our slurm script so we can re-generate it if needed? (This also ensures that the problem is running on the HPC.)
• For bugfixes, it is a robust fix and not a hacky workaround.

[markfuge]

Thanks @g-braeunlich I think this is largely good, except I found a few things:

1. The image is missing (i.e., img/problems/thermoelastic3d.png). Perhaps @gapaza can help you with this?
2. The link to the problem is missing from the right side nav bar. Perhaps we can take an approach similar to HeatConduction where we have boht 2D and 3D problems?

Can @gapaza take a look as well on the technical side to make sure everything is correct and renders properly?

[gapaza]

Taking a look at this today! I will add both the docs and the image as requested.

[gapaza]

Ok, I just added the image and modified the index.md file to include the thermoelastic3d documentation (I also made sure to use the correct docs(project) commit tag as we just discussed). I downloaded the documentation from the GitHub action as Mark showed me and everything looks ok to me.

[g-braeunlich]

Thanks a lot!
@markfuge do we want to merge?

[markfuge]

Yep approved. Merge when ready @g-braeunlich