doc: improve storage drivers reference (canonical#17653)

This PR separates the features comparison table into local and non-local
tables and moves them to the top of the document. The remote, shared,
and object storage driver sections are newly grouped together under a
non-local section, and I renamed the "object storage" driver section to
"object storage backend" to provide differentiation from the "object
storage" feature.

It also expands the features reference section with details about each
feature, adds more information about remote versus shared storage, and
details about the benefit of remote storage over local storage for
instance migration. A legend is included with the tables to explain the
marks used (supported, not supported, not applicable).

## Checklist

- [x] I have read the [contributing
guidelines](https://github.com/canonical/lxd/blob/main/CONTRIBUTING.md)
and attest that all commits in this PR are [signed
off](https://github.com/canonical/lxd/blob/main/CONTRIBUTING.md#including-a-signed-off-by-line-in-your-commits),
[cryptographically
signed](https://github.com/canonical/lxd/blob/main/CONTRIBUTING.md#commit-signature-verification),
and follow this project's [commit
structure](https://github.com/canonical/lxd/blob/main/CONTRIBUTING.md#commit-structure).
- [x] I have checked and added or updated relevant documentation.

Author: tomponline