docs: improve glab attestation commands docs
Description
This merge request improves the documentation and help text for the GitLab CLI attestation feature.
The changes make the documentation clearer by adding better descriptions of what attestations are (signed statements about how software artifacts were built), reorganizing information in a more logical order, and standardizing the formatting of examples.
The updates also add consistent experimental warnings throughout and fix minor text inconsistencies like punctuation and capitalization.
Related issues and merge requests
Documentation
If this MR adds or updates CLI command documentation, follow the conventions in the
GitLab CLI (glab) documentation style guide,
run make gen-docs, and commit the updated files in docs/source/.