Add guidance about migrating away from charming-actions; general guidance about building in CI

[How to set up continuous integration for a charm](https://canonical.com/juju/docs/ops/latest/howto/set-up-continuous-integration-for-a-charm/) mentions `charming-actions`. However, in https://github.com/canonical/operator/pull/2405#discussion_r3013092317 Tony suggested that we figure out for each feature that `charming-actions` offers what the Charm Tech approved alternative is, and then update our docs.

That how-to guide doesn't cover "building" charms at all. I initially opened #2707 to link to the Charmcraft doc about remote builds, but after discussion we decided this needs more thought. See comments in the PR.

Start by reading the continuous-integration guide linked in the issue, then review the discussion in PR #2405 and the context from #2707 about Charmcraft remote builds. Identify an approved alternative for each charming-actions feature and decide what general charm-building guidance belongs in the documentation. Done means the guide no longer relies on charming-actions and clearly covers the agreed build workflow.

Written by the indexing model from the issue text.