PYTHON-5952 Update contributor docs and CI triggers to use main branch

[aclark4life]

PYTHON-5952

Changes in this PR

After renaming the default branch master -> main several self-referencing links and CI branch triggers still pointed at master. This PR updates:

• README.md: codecov badge ?branch=main; contributing-guide link /blob/main/.
• CONTRIBUTING.md: /tree/main/doc link.
• doc/index.rst: contributing-guide link /blob/main/.
• .github/workflows/{test-python,codeql,zizmor,sbom}.yml: push branch triggers changed from master to main.

Test Plan

CI-trigger behavior is exercised when pushes land on main.

Checklist

Checklist for Author

• Did you update the changelog (if necessary)? N/A — docs / CI config, no user-facing behavior.
• Is there test coverage? N/A — see Test Plan.
• Is any followup work tracked in a JIRA ticket? If so, add link(s). Related script fix: PYTHON-5950 (PYTHON-5950 Fix spec resync scripts to use main branch #2951).

Checklist for Reviewer

• Does the title of the PR reference a JIRA Ticket?
• Do you fully understand the implementation? (Would you be comfortable explaining how this code works to someone else?)
• Is all relevant documentation (README or docstring) updated?

[Copilot]

Pull request overview

This PR updates repository documentation links and GitHub Actions workflow triggers to reflect the default branch rename from master to main, preventing stale self-references and ensuring CI runs on the correct branch.

Changes:

• Updated self-referencing GitHub links in README.md, CONTRIBUTING.md, and doc/index.rst to use /main/.
• Updated GitHub Actions push branch filters from master to main across multiple workflows.
• Updated the Codecov badge to track coverage for the main branch.

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
README.md	Updates Codecov badge branch and contributing-guide link to main.
doc/index.rst	Updates contributing-guide link to main.
CONTRIBUTING.md	Updates documentation source link to /tree/main/doc.
.github/workflows/zizmor.yml	Updates push trigger branch filter to main.
.github/workflows/test-python.yml	Updates push trigger branch filter to main.
.github/workflows/sbom.yml	Updates push trigger branch filter to main.
.github/workflows/codeql.yml	Updates push trigger branch filter to main.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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