[Docs/PTX] Add device tensor map init example

[ahendriksen]

Description

closes https://github.com/NVIDIA/cccl/issues/1982

Adds documentation of tensormap modification/initialization on device. Also improves on navigation and table of contents of the cuda::ptx docs.

Checklist

• [x] New or existing tests cover these changes.
• [x] The documentation is up to date with these changes.

[ahendriksen]

Please don't merge yet. I still have to incorporate some internal feedback.

[ahendriksen]

In the mean time, the code example for on-device tensor map modification has made it into the CUDA programming guide. Instead of duplicating the documentation and code sample, I have linked to the relevant section in the programming guide. The improvements to table of contents and layout are still very much worth it in this PR.

I have copied over the fixed links from Bryan van de Ven's PR.

[copy-pr-bot[bot]]

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[ahendriksen]

/ok to test

[ahendriksen]

/ok to test

[miscco]

/ok to test