[DOC] Add policy order to sampling doc

[knylander-grafana]

What this PR does:

Add information about sampling policy order. Written with @ayah-el.

Which issue(s) this PR fixes:
Fixes #

Checklist

• Tests updated
• Documentation added
• CHANGELOG.md updated - the order of entries should be [CHANGE], [FEATURE], [ENHANCEMENT], [BUGFIX]

[github-actions]

💻 Deploy preview deleted.

[tempo-ci-app]

The backport to release-v2.9 failed:

The process '/usr/bin/git' failed with exit code 128

To backport manually, run these commands in your terminal:

# Fetch latest updates from GitHub
git fetch
# Create a new branch
git switch --create backport-5694-to-release-v2.9 origin/release-v2.9
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x a36fddc7ff3e9a42ce06b8b249d7f4390a677289

When the conflicts are resolved, stage and commit the changes:

git add . && git cherry-pick --continue

If you have the GitHub CLI installed:

# Push the branch to GitHub:
git push --set-upstream origin backport-5694-to-release-v2.9
# Create the PR body template
PR_BODY=$(gh pr view 5694 --json body --template 'Backport a36fddc7ff3e9a42ce06b8b249d7f4390a677289 from #5694{{ "\n\n---\n\n" }}{{ index . "body" }}')
# Create the PR on GitHub
echo "${PR_BODY}" | gh pr create --title '[release-v2.9] [DOC] Add policy order to sampling doc' --body-file - --label 'type/docs' --label 'backport' --base release-v2.9 --milestone release-v2.9 --web

Or, if you don't have the GitHub CLI installed (we recommend you install it!):

# Push the branch to GitHub:
git push --set-upstream origin backport-5694-to-release-v2.9

# Create a pull request where the `base` branch is `release-v2.9` and the `compare`/`head` branch is `backport-5694-to-release-v2.9`.

# Remove the local backport branch
git switch main
git branch -D backport-5694-to-release-v2.9