docs: correct default branch prefixes and explain seeding the first tag

[lesnik512]

Summary

The docs pointed users at setups that silently do nothing:

• The GitHub and GitLab provider pages said fix/ bumps patch and feat/ bumps minor. The default branch-prefix config matches feature/ (minor) and bugfix//hotfix/ (patch), so those branches produce no bump.
• The tag output example was v1.2.3. semvertag creates unprefixed tags, and v-prefixed tags fail semver.Version.parse and are skipped.
• Nothing said a first tag must exist. Without one, every run reports no_tags and exits 0, and the reason text cited a stale "v1.0".

Changes

• Provider pages use the real default prefixes and a 1.2.3 example (also in action.yml).
• README, docs/index.md and both provider quick starts say to create a plain semver tag such as 0.1.0 first.
• The no_tags wire reason and the terminal message now say to create an initial tag on a default-branch commit. Both are edited together, per the comment in _outcome.py; tests cover both.
• The README branch-prefix bullet says the prefix is matched in the merge subject, which is what BranchPrefixStrategy.decide does, instead of "starts with".

Checklist

• Lint and format pass (ruff)
• Type check passes (ty)
• Tests pass and new behavior is covered
• Build succeeds (uv build) if packaging or build config changed: n/a
• Repo metadata stays consistent across the three surfaces (GitHub description, pyproject description, profile blurb) if this touches packaging: n/a