docs: Improve link to LXD storage pools

The link to "storage pools" under the [Define storage location](https://canonical-charmed-postgresql.readthedocs-hosted.com/16/how-to/deploy/juju-storage/#define-storage-location) heading at "Deploy on Juju storage - Charmed PostgreSQL 16 documentation" points to an external (non-Canonical) article about LXD storage.

It should pbbly:

- Point to official LXD docs, such as https://documentation.ubuntu.com/lxd/latest/storage/
- Specify that it only links to instructions for LXD storage (i.e. not other types of storage)
- Or, pbbly even better, link to Juju docs that describe storage pools, e.g. https://documentation.ubuntu.com/juju/latest/howto/manage-storage-pools/

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*Reported from: https://canonical-charmed-postgresql.readthedocs-hosted.com/16/how-to/deploy/juju-storage/*