techdocs: cache docs site when built using urlReader for 30 minutes

This caching makes it usable experience, so that docs are not built on every load.
In future readTree will support a method to fetch the timestamp of the latest HEAD. And
it should be used to invalidate the cache.

.changeset/techdocs-rotten-crabs-ring.md

@@ -0,0 +1,5 @@
+---
+'@backstage/plugin-techdocs-backend': patch
+---
+
+If using Url Reader, cache downloaded source files for 30 minutes.