docs: fix stale master branch link in docs overview

[paustint]

Rebuilt on top of current main — the original branch (from April) updated master→main references in ci.yml, CONTRIBUTING.md, README.md, and docs/docs/overview.md, but the first three were fixed independently on main in the meantime.

The one surviving change: the contribution-guidelines link in docs/docs/overview.md still pointed at blob/master, which does not exist. Now points at blob/main.

Docs-only, so no changelog entry is required.

[Copilot]

Pull request overview

This PR updates documentation and CI labeling to reflect the repository’s rename from the master branch to the main branch.

Changes:

• Updated README and docs contribution links from blob/master to blob/main.
• Updated CONTRIBUTING instructions to reference main as the base branch for PRs.
• Updated the GitHub Actions CI workflow step label from “Checkout [master]” to “Checkout [main]”.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
README.md	Updates the CONTRIBUTING link to use blob/main.
docs/docs/overview.md	Updates the CONTRIBUTING link to use blob/main (but still points to a different repo owner).
CONTRIBUTING.md	Updates contributor instructions to use main as the base branch.
.github/workflows/ci.yml	Updates the checkout step display name to reference main.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.