docs: Update Grafana Agent to Grafana Alloy

[JStickler]

What this PR does / why we need it:
This PR updates the documentation to note that Grafana Agent has been deprecated in favor of Grafana Alloy.
Adds note that Promtail is feature complete.
Also fixes a few typos and updates a few cross-references to use full URLs.

Special notes for your reviewer:
I left the references to Grafana Agent in the following places:

• Helm charts documentation, because I don't think we've updated the charts to use Alloy yet.
• Migration documentation, as it is likely that users who are migrating would still be using the Grafana Agent.

{{< docs/shared source="alloy" lookup="agent-deprecation.md" version="next" >}} Will pull in a shared file from the Alloy docs that inserts the following Caution. (Screen shot from Tempo docs, only the Caution will be imported)

[MichelHollands]

LGTM

[grafanabot]

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

[grafanabot]

The backport to release-3.0.x failed:

The process '/usr/bin/git' failed with exit code 1

To backport manually, run these commands in your terminal:

# Fetch latest updates from GitHub
git fetch
# Create a new branch
git switch --create backport-12602-to-release-3.0.x origin/release-3.0.x
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x 74a5bbc5caa3cea306aa7047b73fb81738d80872

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-12602-to-release-3.0.x
# Create the PR body template
PR_BODY=$(gh pr view 12602 --json body --template 'Backport 74a5bbc5caa3cea306aa7047b73fb81738d80872 from #12602{{ "\n\n---\n\n" }}{{ index . "body" }}')
# Create the PR on GitHub
echo "${PR_BODY}" | gh pr create --title 'chore: [release-3.0.x] docs: Update Grafana Agent to Grafana Alloy' --body-file - --label 'size/M' --label 'type/docs' --label 'backport' --base release-3.0.x --milestone release-3.0.x --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-12602-to-release-3.0.x

# Create a pull request where the `base` branch is `release-3.0.x` and the `compare`/`head` branch is `backport-12602-to-release-3.0.x`.

# Remove the local backport branch
git switch main
git branch -D backport-12602-to-release-3.0.x