docs: cover GitHub on the index page; branch-prefix reads the head commit

[lesnik512]

Summary

Two doc inaccuracies:

• docs/index.md described semvertag as a GitLab tool ("Auto-tag your GitLab repository", "from your GitLab project") and showed only a GitLab quick start, although GitHub is a supported provider (semvertag/providers/github.py, docs/providers/github.md). The intro now names both providers, and the quick start has a GitHub Actions snippet (copied from docs/providers/github.md) next to the GitLab one. The first-tag note moves above both snippets, and the local-run example is labelled as GitLab.
• docs/strategies/branch-prefix.md said the strategy inspects "the subject line of each commit", and the index said "the latest merge commit". The strategy decides on one commit: get_latest_commit_on_default_branch() in each provider feeds BranchPrefixStrategy.decide(commit). Both pages now say it reads the head commit on the default branch, which must be a merge commit.

Checks

• just lint-ci passes
• just test: 475 passed
• mkdocs build --strict passes