Move and reword blue-print monitor-workflows

https://freexian-team.pages.debian.net/debusine/reference/devel-blueprints/monitor-workflows.html : implemented parts should be removed, and should move (possibly) the "Runtime status of the workflow" section into a new home.

Also add the newsfragment/server/400.feature.rst with something such as:

"New view with list of workflows (/<scope>/<workspace>/workflow/). List workflow templates with stats in the workspace view (</scope></workspace>/), new view with specific template information (/<scope>/<workflow-template>/<name>/).