Add alt text and fix broken image links

[clayton-cornell]

PR Description

Doc build reports show missing alt text and several broken image links. This PR adds the missing alt text and corrects the bad/broken image links.

Which issue(s) this PR fixes

Notes to the Reviewer

PR Checklist

• [ ] CHANGELOG.md updated
• [ ] Documentation added
• [ ] Tests updated
• [ ] Config converters updated

[grafanabot]

The backport to release-v0.40 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-6955-to-release-v0.40 origin/release-v0.40
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x a834fa5ce6116d06ada51f6bd76ae77d14c7ab61
# Push it to GitHub
git push --set-upstream origin backport-6955-to-release-v0.40
git switch main
# Remove the local backport branch
git branch -D backport-6955-to-release-v0.40

Then, create a pull request where the base branch is release-v0.40 and the compare/head branch is backport-6955-to-release-v0.40.