serve s3: pre-merge tweaks

- Changes
    - Rename `--s3-authkey` to `--auth-key` to get it out of the s3 backend namespace
    - Enable `Content-MD5` integrity checks
    - Remove locking after code audit
- Documentation
    - Factor out documentation into seperate file
    - Add Quickstart to docs
    - Add Bugs section to docs
    - Add experimental tag to docs
    - Add rclone provider to s3 backend docs
- Fixes
    - Correct quirks in s3 backend
    - Change fmt.Printlns into fs.Logs
    - Make metadata storage per backend not global
    - Log on startup if anonymous access is enabled
- Coding style fixes
    - rename fs to vfs to save confusion with the rest of rclone code
    - rename db to b for *s3Backend

Fixes #7062

cmd/serve/s3/server.go

@@ -45,6 +45,10 @@ func newServer(ctx context.Context, f fs.Fs, opt *Options) (s *Server, err error
 		vfs: vfs.New(f, &vfsflags.Opt),
 	}
 
+	if len(opt.authPair) == 0 {
+		fs.Logf("serve s3", "No auth provided so allowing anonymous access")
+	}
+
 	var newLogger logger
 	w.faker = gofakes3.New(
 		newBackend(w.vfs, opt),
@@ -53,6 +57,7 @@ func newServer(ctx context.Context, f fs.Fs, opt *Options) (s *Server, err error
 		gofakes3.WithRequestID(rand.Uint64()),
 		gofakes3.WithoutVersioning(),
 		gofakes3.WithV4Auth(authlistResolver(opt.authPair)),
+		gofakes3.WithIntegrityCheck(true), // Check Content-MD5 if supplied
 	)
 
 	w.Server, err = httplib.NewServer(ctx,