split per-type registration points into per-type files

- move README per-type sections into docs/<type>/{remote,local,virtual}.md
- split provider.go capability and metadata types into cap_*.go and metadata_*.go
- register providers via internal/provider/all, one file per type
- declare each package type in its own file, registering via init()
- split the e2e caching table and UI type colours/snippets per type

e2e-docker/caching_docker_test.go

@@ -0,0 +1,7 @@
+//go:build dockere2e
+
+package e2edocker
+
+func init() {
+	cachingCases = append(cachingCases, cachingCase{"docker", "library/testimg/blobs/blobdata", "v2/library/testimg/blobs/blobdata"})
+}