[DOC] Add note for Agent to Alloy to docs

[knylander-grafana]

With the release of Grafana Alloy, we're adding an admonition to the Agent-related documenation notifying people about Alloy and the deprecation of Agent.

This admonition is contained in a shared file that is in the Alloy repository. When used, the admonition looks like this:

[github-actions]

This PR must be merged before a backport PR will be created.

[github-actions]

This PR must be merged before a backport PR will be created.

[simonswine]

LGTM

[github-actions]

The backport to release/v1.5 failed:

Validation Failed: "Could not resolve to a node with the global id of 'T_kwDOAG3Mbc4AczmP'."

To backport manually, run these commands in your terminal:

# Fetch latest updates from GitHub
git fetch
# Create a new branch
git switch --create backport-3201-to-release/v1.5 origin/release/v1.5
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x 4cb01d6197c682ab006d033d69d1174e60397bf8

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-3201-to-release/v1.5
# Create the PR body template
PR_BODY=$(gh pr view 3201 --json body --template 'Backport 4cb01d6197c682ab006d033d69d1174e60397bf8 from #3201{{ "\n\n---\n\n" }}{{ index . "body" }}')
# Create the PR on GitHub
echo "${PR_BODY}" | gh pr create --title "[release/v1.5] [DOC] Add note for Agent to Alloy to docs" --body-file - --label "type/docs" --label "backport" --base release/v1.5 --milestone release/v1.5 --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-3201-to-release/v1.5

# Create a pull request where the `base` branch is `release/v1.5` and the `compare`/`head` branch is `backport-3201-to-release/v1.5`.

# Remove the local backport branch
git switch main
git branch -D backport-3201-to-release/v1.5