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/s3_test.go

@@ -69,7 +69,6 @@ func TestS3(t *testing.T) {
 			"type":              "s3",
 			"provider":          "Rclone",
 			"endpoint":          testURL,
-			"list_url_encode":   "true",
 			"access_key_id":     keyid,
 			"secret_access_key": keysec,
 		}