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

backend/s3/s3.go

@@ -3134,6 +3134,13 @@ func setQuirks(opt *Options) {
 		// No quirks
 		useMultipartEtag = false // untested
 		useAlreadyExists = false // untested
+	case "Rclone":
+		listObjectsV2 = true
+		urlEncodeListings = true
+		virtualHostStyle = false
+		useMultipartEtag = false
+		useAlreadyExists = false
+		// useMultipartUploads = false - set this manually
 	case "Scaleway":
 		// Scaleway can only have 1000 parts in an upload
 		if opt.MaxUploadParts > 1000 {
@@ -3152,11 +3159,6 @@ func setQuirks(opt *Options) {
 		virtualHostStyle = false
 		urlEncodeListings = false
 		useAlreadyExists = false // untested
-	case "Rclone":
-		listObjectsV2 = true
-		urlEncodeListings = true
-		virtualHostStyle = false
-		useMultipartEtag = false
 	case "Storj":
 		// Force chunk size to >= 64 MiB
 		if opt.ChunkSize < 64*fs.Mebi {