Replace < with % in var references in headings

[clayton-cornell]

PR Description

The table of contents is generated during Markdown processing and shortcodes with {{< VAR_NAME >}} syntax haven't been rendered at that point.

Recent changes to use a cascading variable for the Grafana Agent Flow product name use the {{< VAR_NAME >}} shortcode in many of the headings. The result is the heading anchor link URLs contain hahahugoshortcodes0hbhb in place of Grafana Agent Flow

To fix this we have to use {{% VAR_NAME %}} shortcode syntax in the headings only. This tells Hugo to render the shortcode before the markdown... and... anchor link URLs look normal.

Which issue(s) this PR fixes

Notes to the Reviewer

PR Checklist

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

[jdbaldry]

❤️

[grafanabot]

The backport to release-v0.38 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-5975-to-release-v0.38 origin/release-v0.38
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x 7e0bf25ed090a92d235b506aa021605be7f25e0f
# Push it to GitHub
git push --set-upstream origin backport-5975-to-release-v0.38
git switch main
# Remove the local backport branch
git branch -D backport-5975-to-release-v0.38

Then, create a pull request where the base branch is release-v0.38 and the compare/head branch is backport-5975-to-release-v0.38.