Update documentation and CI configuration to use 'main' branch

[paustint]

Change all references from 'master' to 'main' in documentation and CI configuration to reflect the new branch naming convention.

[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.

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[Copilot]

This link points to the paustint/soql-parser-js repository, but this package/repo identifies as jetstreamapp/soql-parser-js (see package.json repository URL). Update the contribution-guidelines link to the correct repository (or use a relative CONTRIBUTING.md link) so contributors don’t land on the wrong project.

Suggested change

	All contributions are welcome on the project. Please read the [contribution guidelines](https://github.com/paustint/soql-parser-js/blob/main/CONTRIBUTING.md).
	All contributions are welcome on the project. Please read the [contribution guidelines](./CONTRIBUTING.md).

[Copilot]

The list item starts with lowercase "open". For consistency with the surrounding imperative bullets, capitalize it (e.g., "Open a pull request...") and consider using consistent casing for "pull request" throughout the doc.

Suggested change

	- open a Pull Request for the main branch
	- Open a pull request for the main branch