Update onboarding instructions in CONTRIBUTING.md

[Tomlu60220244]

Fixes #8551

What changes did you make?

• Updated CONTRIBUTING.md Section 1.1 onboarding instructions
• Replaced the Slack-first repository access instructions with onboarding-form-first instructions

Why did you make the changes (we will use this info to test)?

• To reduce confusion for contributors who already receive repository access through onboarding

For Reviewers: Do not review changes locally, rather review changes at:
https://github.com/Tomlu60220244/website/blob/update-contributing-8551/CONTRIBUTING.md

Screenshots of Proposed Changes To The Website (if any, please do not include screenshots of code changes)

• No visual changes to the website. This PR updates CONTRIBUTING.md documentation only.

This PR replaces closed PR #8597, which was originally opened from a branch that did not include the issue number in its name.

[github-actions]

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Note that CONTRIBUTING.md cannot previewed locally; rather it should be previewed at this URL:

https://github.com/Tomlu60220244/website/blob/update-contributing-8551/CONTRIBUTING.md  

[Tomlu60220244]

Hi reviewers — this PR replaces #8597 so the branch name now includes the issue number as requested. Please continue review here. Thank you.

[viet456]

Thanks for adding the issue number to your branch. Everything looks good.

[jjam02]

Everything looks great thanks for taking on this issue

[kdaca19xx]

Great effort Tomlu60220244!

Things Done Well

• The pull request was done with the correct branch.

• There's a linked issue. I read/understood it.

• Your edits in https://github.com/Tomlu60220244/website/blob/update-contributing-8551/CONTRIBUTING.md look good. (no need to check changes using Docker)

Suggestions

none. Thx again, Tomlu60220244!