docs: add v1.21 release notes

[marcsanmi]

Summary

• Adds curated release notes for Pyroscope 1.21.0 to the docs site.
• Mirrors the format of v1-20.md: enhancements, fixes, security fixes, documentation.
• v1.21.0 shipped on GitHub on 2026-04-17 but the grafana.com docs site has no entry for it; this fills that gap.

Source of truth cross-checked against the v1.21.0 GitHub release and `git log v1.20.4..v1.21.0`.

Test plan

• Verify docs build renders `v1-21.md` in the release-notes section.
• Confirm no PRs from the auto-generated GitHub changelog are missing that should be in the curated list (meta/doc-only/CI/chart-bump/renovate commits intentionally skipped).

🤖 Generated with Claude Code

---

Note

Low Risk
Docs-only change that adds a new markdown release-notes page; no runtime code or configuration behavior is modified.

Overview
Adds a new docs/sources/release-notes/v1-21.md page documenting the Grafana Pyroscope 1.21.0 release.

The page follows the existing release-notes structure and curates the key Enhancements, Fixes, Security fixes, and Documentation items with links to the corresponding PRs/changelog comparison.

^{Reviewed by Cursor Bugbot for commit 82b8ba3. Bugbot is set up for automated code reviews on this repo. Configure here.}

[pyroscope-development-app]

Hello @marcsanmi!
Backport pull requests need to be either:

• Pull requests which address bugs,
• Urgent fixes which need product approval, in order to get merged,
• Docs changes.

Please, if the current pull request addresses a bug fix, label it with the type/bug label.
If it already has the product approval, please add the product-approved label. For docs changes, please add the type/docs label.
If the pull request modifies CI behaviour, please add the type/ci label.
If none of the above applies, please consider removing the backport label and target the next major/minor release.
Thanks!