chore(docs): Add docs for how to publish a release for previous majors (#14740)

Backporting the release docs for older branches.

Author: andreiborza

docs/assets/run-release-workflow.png

@@ -20,6 +20,21 @@ _These steps are only relevant to Sentry employees when preparing and publishing
    [@getsentry/releases-approvers](https://github.com/orgs/getsentry/teams/release-approvers) to approve the release. a.
    Once the release is completed, a sync from `master` ->` develop` will be automatically triggered
 
+## Publishing a release for previous majors
+
+1. Run `yarn changelog` on the major branch (e.g. `v8`) and determine what version will be released (we use
+   [semver](https://semver.org))
+2. Create a branch, e.g. `changelog-8.45.1`, off the major branch (e.g. `v8`)
+3. Update `CHANGELOG.md` to add an entry for the next release number and a list of changes since the
+   last release. (See details below.)
+4. Open a PR with the title `meta(changelog): Update changelog for VERSION` against the major branch.
+5. Once the PR is merged, open the [Prepare Release workflow](https://github.com/getsentry/sentry-javascript/actions/workflows/release.yml) and
+   fill in ![run-release-workflow.png](./assets/run-release-workflow.png)
+   1. The major branch you want to release for, e.g. `v8`
+   2. The version you want to release, e.g. `8.45.1`
+   3. The major branch to merge into, e.g. `v8`
+6. Run the release workflow
+
 ## Updating the Changelog
 
 1. Run `yarn changelog` and copy everything.