This commit introduces release automation triggered by button clicks in
Github Actions, guarded by a check on whether all the Cargo.toml files
contain the same version string.
On PRs, changes to documentation no longer trigger code tests.
Similarly, changes to code that don't update documentation do not
trigger documentation tests. Changes that fail at the `cargo check`
stage abort early to prevent lengthy CI builds of the installer and
firmware.
Commits on the `main` branch always run the full test suite regardless
of what changed.
Releases also run the full check, test, build and publish suite.
