Improve the httpbin charm and document it

[dwilding]

There are several things to improve about the httpbin charm that we're adding to Ops in #1743. It's probably best to create individual issues & PRs for these improvements when working on them.

• Switch to uv and tox-uv? Or is there value in having an example charm that isn't uv-based?

• Add tox environments for formatting and linting? I removed these when I ported the charm from Charmcraft because the Ops repo has its own formatting and linting, but perhaps it's better for the charm to define them. (I actually forgot to remove format and lint from the list of default environments in tox.ini)

• Improve how we're returning a layer. See https://github.com/canonical/charmcraft/issues/1116

• Switch the integration test to Jubilant.

• Look at switching to a different workload. As Dima observed, the current workload is 6 years old and only supports AMD64.

• See whether there's a suitable place in the Ops docs to mention the charm. Currently it's used as an example in the README, which we might want to change. See #1655.

Also: #1839