Investigate subordinate charms and update the doc

In https://github.com/canonical/operator/pull/2455 we're adding a summary doc about subordinate charms. While researching this doc, we identified several questions that may need to be answered in a future version of the doc:

1. What data can subordinate units see? Is there a peer relation with other units of the same subordinate charm? [How to manage relations](https://documentation.ubuntu.com/ops/latest/howto/manage-relations/) has some information about relations that involve a subordinate charm. One part of that doc says "subordinate units cannot see each other’s peer data", which I don't understand.

2. Does Juju block relations if the bases are incompatible? Presumably when you deploy the subordinate, Juju wouldn't complain about anything because it doesn't know what you are going to integrate with - at this point no units of the subordinate charm are created.

3. Do we want our own example of a subordinate charm?

Answering these and updating the doc will require some testing and discussion with people who have more knowledge about subordinate charms.

Start with pull request #2455 and the linked “How to manage relations” documentation. Investigate the three subordinate-charm questions through testing and discussion with Juju or charm experts, then update the summary documentation with the answers and any agreed example.

Written by the indexing model from the issue text.