Add symmetric tensor tutorial to docs as appendix

[leburgel]

Adds the symmetric tensor tutorial from the quantumghent tensor tutorials to the docs as an appendix, as discussed in #289.

In the process, I add subblock and subblocks to the library docs, since these were missing before so I couldn't reference their docstrings.

[codecov]

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[github-actions]

After the build completes, the updated documentation will be available here

[leburgel]

Not really the main point here, but I would also prefer it if all toc depths in docs/src/index.md were set to 2. I've always thought the depth-3 ones look very busy.

[lkdvos]

Will try and go through in more detail, some small comments:

It might make the notation a bit lighter if we replace ComplexF64 with Float64 for the cases where this is applicable, I know we don't default to this in MPSKit for various reasons, but here it seems a bit much to me.

For git reasons it might be convenient to break lines at sentences instead of splitting at linewidth. It also tends to make paragraphs less jumpy for small changes/comments

[Jutho]

Great tutorial. I left some minor comments and suggestions.

[lkdvos]

Left some final comments but otherwise definitely good to go for me. Great work, thank you!

[Jutho]

Ok, I think these are the final suggestions. If these are addressed, I will approve and merge.