Remove references to milestone & remove it from project management docs

[kaixin-hc]

What is the purpose of this pull request?

• Documentation update
• Bug fix
• Feature addition or enhancement
• Code maintenance
• DevOps
• Improve developer experience
• Others, please explain:

Resolves #2142

Overview of changes:
Remove references to milestone and steps relating to milestone from our DG. Also added a short tip to the bottom about why this was removed so future developers understand our reasoning.

Anything you'd like to highlight/discuss:

Testing instructions:
Go to the project management page in netlify and read through it.

Proposed commit message: (wrap lines at 72 characters)
Remove milestones from our project workflow docs

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Checklist: ☑️

• Updated the documentation for feature additions and enhancements
• Added tests for bug fixes or features
• Linked all related issues
• No unrelated changes

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Reviewer checklist:

Indicate the SEMVER impact of the PR:

• Major (when you make incompatible API changes)
• Minor (when you add functionality in a backward compatible manner)
• Patch (when you make backward compatible bug fixes)

At the end of the review, please label the PR with the appropriate label: r.Major, r.Minor, r.Patch.

Breaking change release note preparation (if applicable):

• To be included in the release note for any feature that is made obsolete/breaking

Give a brief explanation note about:

• what was the old feature that was made obsolete
• any replacement feature (if any), and
• how the author should modify his website to migrate from the old feature to the replacement feature (if possible).

[yucheng11122017]

LGTM except small nit! Thank you:)

[github-actions]

@kaixin-hc Each PR must have a SEMVER impact label, please remember to label the PR properly.