Create CHANGELOG.md

[IllusionMH]

PR checklist

• Addresses an existing issue: fixes No changelog #654
• Documentation update

Overview of change:

• New CHANGELOG.md created with all data from release notes.
• Releases for 6.0.0 and 6.0.0-beta tags created
• Updated 6.0.0 milestone to include issues/PRs that made it to the release

TODO:

• Create releases back to 5.0.0 and link them in CHANGELOG.md
• Update Releases wiki page (before Move documentation from the wiki into ./docs #808)
• Remove Replace Release Notes wiki page

Is there anything you'd like reviewers to focus on?

Changed Internal source code improvements to Internal improvements because they might include infrastructure and dependencies changes.

Do we need to have some header in CHANGELOG.md?
Like "Change Log" in TSLint or "Unreleased" in React repo (although doesn't looks like it is used there).

[JoshuaKGoldberg]

Changed Internal source code improvements to Internal improvements because they might include infrastructure and dependencies changes.

👍. Perfection is when there is nothing left to remove...

Do we need to have some header in CHANGELOG.md?

# Changelog SGTM, given the file title.

Approving changes so far; up to you if you want to tackle the to-do list in this PR or continue it after.

[IllusionMH]

Updated Releases page to mention CHANGELOG.md and GitHub Releases
Replaced Release Notes content with links to CHANGELOG.md and GitHub Releases so anyone who used that page will know where to find latest release notes.

Can be merged to close issue

[JoshuaKGoldberg]

Fantastic, thanks @IllusionMH!

Somehow I don't think many folks are going to be too interested in versions prior to 5. 😉