[DOC] Update metrics query docs with examples, more details

[knylander-grafana]

What this PR does:

Second try. Original PR with review comments: #4128

This doc has a first draft of a use case for using TraceQL metrics queries to investigate an issue. The example provides equivalent PromQL and TraceQL queries.

Updates the TraceQL metrics documentation to add:

• Create a separate functions page with the functions moved off of the main metrics page
• Maintain a list of the functions on the main TraceQL metrics page
• Add a section that describes how RED metrics can be generated in TraceQL metrics vs span metrics from the metrics generator
• Add examples for PromQL vs TraceQL equivalent queries - how do you contruct a TraceQL query that is equivalent of a PromQL query

Which issue(s) this PR fixes:
Fixes https://github.com/grafana/tempo-squad/issues/413

Checklist

• Tests updated
• Documentation added
• CHANGELOG.md updated - the order of entries should be [CHANGE], [FEATURE], [ENHANCEMENT], [BUGFIX]

[knylander-grafana]

This admonition is new and needs to be reviewed.

[github-actions]

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

[github-actions]

The backport to release-v2.6 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-4248-to-release-v2.6 origin/release-v2.6
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x 532f83f9338542c4f63052b104e6c1205e8b6f36

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-4248-to-release-v2.6
# Create the PR body template
PR_BODY=$(gh pr view 4248 --json body --template 'Backport 532f83f9338542c4f63052b104e6c1205e8b6f36 from #4248{{ "\n\n---\n\n" }}{{ index . "body" }}')
# Create the PR on GitHub
echo "${PR_BODY}" | gh pr create --title '[release-v2.6] [DOC] Update metrics query docs with examples, more details' --body-file - --label 'type/docs' --label 'backport' --base release-v2.6 --milestone release-v2.6 --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-4248-to-release-v2.6

# Create a pull request where the `base` branch is `release-v2.6` and the `compare`/`head` branch is `backport-4248-to-release-v2.6`.

# Remove the local backport branch
git switch main
git branch -D backport-4248-to-release-v2.6