5.0 GA: update release notes, experimental features, roadmap (#5147)

[ti-srebot]

cherry-pick #5147 to release-5.0
You can switch your code base to this Pull Request by using git-extras:

# In docs repo:
git pr https://github.com/pingcap/docs/pull/5247

After apply modifications, you can push your change to this PR via:

git push git@github.com:ti-srebot/docs.git pr/5247:release-5.0-a86c64b6540a

---

First-time contributors' checklist

• I've signed Contributor License Agreement that's required for repo owners to accept my contribution.

What is changed, added or deleted? (Required)

Which TiDB version(s) do your changes apply to? (Required)

• master (the latest development version)
• v5.0 (TiDB 5.0 versions)
• v4.0 (TiDB 4.0 versions)
• v3.1 (TiDB 3.1 versions)
• v3.0 (TiDB 3.0 versions)
• v2.1 (TiDB 2.1 versions)

What is the related PR or file link(s)?

• This PR is translated from: Update 5.0 GA release notes and experimental features docs-cn#5835, Update roadmap docs-cn#5851
• Other reference link(s):

Do your changes match any of the following descriptions?

• Delete files
• Change aliases
• Need modification after applied to another branch
• Might cause conflicts after applied to another branch

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• yikeke

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[yikeke]

/merge

[ti-chi-bot]

This pull request has been accepted and is ready to merge.

Commit hash: fab72dc