Add docs for current backport labels and process. (#10317)

* Capture the basics of backporting process

* broken linK

* Update docs/backports.md

Co-authored-by: Paolo Chilà <[email protected]>

---------

Co-authored-by: Paolo Chilà <[email protected]>

Author: ebeahan

CONTRIBUTING.md

@@ -63,3 +63,6 @@ Keep these in mind as both authors and reviewers of PRs:
 * If I'm making a suggestion, I must provide a rationale for it.  It should be clear to the author of the PR why my suggestion is better than what is already in the PR.
 
 
+## Backporting
+
+See [Backports](./docs/backports.md).

docs/backports.md

@@ -0,0 +1,31 @@
+# Backports
+
+## Overview
+
+Elastic Agent relies on backporting PRs to incorporate changes into the appropriate release branches after merging
+into `main`. Creating backport PRs is handled automatically using the appropriate [backport labels](https://github.com/elastic/elastic-agent/labels?q=backport).
+
+To target specific release branch(es), add one or more labels `backport-<release branch name>` (e.g. `backport-9.2`). PRs can be labeled
+before or after the PR is merged. If added before merging, the backports will be created after merging to `main`. Adding
+a backport label to an already merged PR will create backports shortly after adding the label(s).
+
+Mergify will automatically add a [reminder](https://github.com/elastic/elastic-agent/blob/main/.mergify.yml#L173) to any PR to `main` if missing at least one `backport-*` label. If your PR does _not_ need backporting, use the `backport-skip` label to avoid the automated comment.
+
+There are also three additional `backport-*` labels to simplify which branches to target backports:
+
+* `backport-active-all`: Backport to all currently active development branches (except `main`)
+* `backport-active-8`: Backport only to 8.x branches
+* `backport-active-9`: Backport only to 9.x branches
+
+
+These labels use the [`backport-active` GitHub action](https://github.com/elastic/elastic-agent/blob/main/.github/workflows/backport-active.yml#L14) to automate targeting relevant branches. More detail can be found at
+https://github.com/elastic/oblt-actions/blob/main/github/backport-active/README.md.
+
+## Process
+
+**For changes authored by an individual** - the original PR author is in charge of ensuring their original PR uses the
+correct labels, the backport PRs are created, pass CI, and are merged.
+
+**For changes authored by automation (GitHub actions, dependabot, etc.)** - these PRs should already be labeled with
+`backport-active-all`. After the initial PR to `main` merges, the code reviewers (assigned based on teams defined in
+`CODEOWNERS`) need to verify CI passes, approve, and merge the backport PRs.