Re-add toc navigation

[bmunkholm]

Summary of the changes / Why this is an improvement

Adding TOC expand-collapse icons again (as #664 was reverted).
Includes removal of build warnings that was causing the issues of past (reverted) builds. But now working correctly.

This has been tested with both the docs in the theme and the guide.

Preview

https://crate-docs-theme--681.org.readthedocs.build/en/681/

[amotl]

Thanks. If it's possible to remove the SASS updates, the patch will be much more concise, and will not introduce any side effects.

[amotl]

Is it a good idea to change anything on vendorized resources? Instead, I think it's better to keep them unmodified, but regularly update them from the original sources. This remark also applies to all other resources under the vendor/furo directory.

[amotl]

Following this advice, I think it's better to remove 9fe0d52 from this patch again. Maybe you want to submit this to Furo upstream beforehand?

Not following this advice will make it nearly impossible to update corresponding resources on future releases of Furo through updates like GH-677, which can easily introduce changes that require updating corresponding assets. Unfortunately, we can't pull them from the original project dynamically, so they have been included here.

[bmunkholm]

You are absolutely right that that is the "right" way to do it.

The issue was that the original patch for the icons depended on the changes done already for removing the deprecation warnings. So I gave it a try to get rid of all those warnings anyway. The updates to do that are relatively simple to do with the automatic upgrade tool provided: sass-migrator. So even if changes are done to Furo, we can easily apply the same upgrades I did by running that tool on top.
But I get the principle point and generally agree. I'll give it a go to see if I get easily get the icon patch updated to work on top of current main instead.

[bmunkholm]

Alright - that was easy: #682 includes just the nav icons now.