[DOC] Update index and java for Alloy

[knylander-grafana]

Starts updating the Agent documentation for Alloy.

This PR addresses the Grafana Agent _index, Java, and Golang push docs.

Down the road, the Agent docs will be removed or at least moved to the bottom of the pages in a collapsed section.

Related to #3271

[clayton-cornell]

Same question nere... this spot missed (legacy), but I'm thinking both could be dropped once it's stated up at the top of the topic.

[korniltsev]

We may want to mention, that for alloy, users need extra argument --stability.level=public-preview

[clayton-cornell]

We may want to mention, that for alloy, users need extra argument --stability.level=public-preview

Good point. Both pyroscope.write and pyroscope.scrape are in Public preview. If you don't set the stability.level the component will not be available (Alloy defaults to GA only).

There's a note with all Public preview components... documented here: https://grafana.com/docs/alloy/latest/reference/cli/run/#permitted-stability-levels that might be important to link to?

[knylander-grafana]

We may want to mention, that for alloy, users need extra argument --stability.level=public-preview

Would users need to use this flag every time they start Alloy? Is this because Golang pull for Agent/Alloy is considered public preview?

[clayton-cornell]

We may want to mention, that for alloy, users need extra argument --stability.level=public-preview

Would users need to use this flag every time they start Alloy? Is this because Golang pull for Agent/Alloy is considered public preview?

Users would only need this if they are using components flagged as public-preview or experimental. If you're documenting how to use specific Alloy components like pyroscope.scrape, you can look at the Reference docs here: https://grafana.com/docs/alloy/latest/reference/components/ and look at the specific components... so for pyroscope.scrape you go here: https://grafana.com/docs/alloy/latest/reference/components/pyroscope.scrape/ and right at the top there's a box noting it's in Public Preview. The only way to access this then is to add the --stability.level=public-preview argument. Anything GA doesn't need this flag. And... once pyroscope.scrape goes GA, the arguments won't be required anymore.

[knylander-grafana]

@simonswine and @clayton-cornell Would you be willing to re-review?

[github-actions]

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

[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-3272-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 b58be939077b84385d3752c9588f0f8ca1975171

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-3272-to-release/v1.5
# Create the PR body template
PR_BODY=$(gh pr view 3272 --json body --template 'Backport b58be939077b84385d3752c9588f0f8ca1975171 from #3272{{ "\n\n---\n\n" }}{{ index . "body" }}')
# Create the PR on GitHub
echo "${PR_BODY}" | gh pr create --title "[release/v1.5] [DOC] Update index and java for Alloy" --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-3272-to-release/v1.5

# Create a pull request where the `base` branch is `release/v1.5` and the `compare`/`head` branch is `backport-3272-to-release/v1.5`.

# Remove the local backport branch
git switch main
git branch -D backport-3272-to-release/v1.5