Update doc theme to the latest

[Nayef211]

• Deleting outdated docs on experimental datasets
• Update pytorch_sphinx_theme to latest commit
• This also updates torchtext docs with new Terms / Privacy links pointing to Linux foundation

Note we still pin the theme version so that our customization does not break randomly similar to pytorch/audio#2679

[mthrok]

The intended change looks good but there are some other render issues observed.

Follow up:

• pin versions of Sphinx-related tools (example)
• Override CSS so that example gallery is rendered properly.

nit: I'd have split the PR to deletion and theme update.

[Nayef211]

Follow up:

• pin versions of Sphinx-related tools (example)
• Override CSS so that example gallery is rendered properly.

Thanks for the feedback. Let me create a followup PR to do this.

nit: I'd have split the PR to deletion and theme update.

I think the reason I did this in one is because @svekars mentioned the following

To update torchtext docs to pick up this change, you need to rebuild your docs. It can be a super simple PR that just changes a word or fixes a typo under the docs/source dir.

In hindsight, maybe the update to sphinx-theme was enough to rebuild the docs.