doc: Improve assumeutxo guide and add more docs/comments

Summary:
This new doc will be available on the bitcoinabc.org/doc/assumeutxo.html URL after the next deployment of the website, and can be given to users.
When can add a link to snapshot files when the hosting is sorted out (see TODO note left in the document).

I added a section to document the current Chronik incompatibility, similar to what is in the release notes.

Also fixes some outdated information in the remaining design doc.

https://github.com/bitcoin/bitcoin/pull/29553/commits/e868a6e070a91c00555e72181f9b14bbf0373fdc

Add -rpcclienttimeout=0 to loadtxoutset examples

https://github.com/bitcoin/bitcoin/pull/30808/commits/c5eaae3b89c67c664c24a53908bea608785e481d

fix assumeutxo design doc link

This concludes backport of core#29553, core#30808 and core#30819
Depends D18125

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D18126