Document suspending and resuming the Azure resources

[jtopgi]

Why

Asked: "can we shutdown/suspend so we aren't using azure credits, i don't need this app right now."

TransitHike's Azure resources cost about $34 a month: the B1 App Service plan (about $13), the PostgreSQL flexible server (about $16) and the Basic container registry (about $5). Stopping the web app alone doesn't stop the plan's billing, and a stopped flexible server starts itself again after 7 days. So the site is now suspended like this, done with the Azure CLI rather than in this PR:

• The web app is stopped, with Always On off, on the Free (F1) plan. That costs nothing and keeps transithike.azurewebsites.net and its settings.
• The database and registry are deleted, along with the app's DATABASE_URL setting, so bin/azure-setup writes a new one. Nothing in them is lost for good: the cache rebuilds, hiking routes are re-imported from the routes-data release, and a deploy rebuilds the image.
• The Guides workflow is disabled, since its weekly run deploys.
• What's left costs nothing: the stopped app, the Free plan, two managed identities, and Application Insights with its workspace, which now gets no new data.

What this changes

A README section, Suspending and resuming, under Operations. It lists the commands above and how to resume. The plan must go back to B1 before bin/azure-setup, because the script turns on Always On, which the Free plan lacks, and under set -euo pipefail that error would stop the script. Then start the app, run CI to deploy, and enable the Guides workflow.

It merges with [skip ci]: while suspended, a deploy from master fails at az acr login.