docs(aio): document preview server HTTP status codes

aio/aio-builds-setup/docs/overview--http-status-codes.md

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+# Overview - HTTP Status Codes
+
+
+This is a list of all the possible HTTP status codes returned by the nginx anf upload servers, along
+with a bried explanation of what they mean:
+
+
+## `http://*.ngbuilds.io/*`
+
+- **307 (Temporary Redirect)**:
+  All non-HTTPS requests. 308 (Permanent Redirect) would be more appropriate, but is not supported
+  by all agents (e.g. cURL).
+
+
+## `https://pr<pr>-<sha>.ngbuilds.io/*`
+
+- **200 (OK)**:
+  File was found or URL was rewritten to `/index.html` (i.e. all paths that have no `.` in final
+  segment).
+
+- **403 (Forbidden)**:
+  Trying to access a sub-directory.
+
+- **404 (Not Found)**:
+  File not found.
+
+
+## `https://ngbuilds.io/create-build/<pr>/<sha>`
+
+- **201 (Created)**:
+  Build deployed successfully and is publicly available.
+
+- **202 (Accepted)**:
+  Build not automatically verifiable. Stored for later deployment (after re-verification).
+
+- **400 (Bad Request)**:
+  No payload.
+
+- **401 (Unauthorized)**:
+  No `AUTHORIZATION` header.
+
+- **403 (Forbidden)**:
+  Unable to verify build (e.g. invalid JWT token, or unable to talk to 3rd-party APIs, etc).
+
+- **404 (Not Found)**:
+  Tried to change PR visibility but the source directory did not exist.
+  (Currently, this can only happen as a rare race condition during build deployment.)
+
+- **405 (Method Not Allowed)**:
+  Request method other than POST.
+
+- **409 (Conflict)**:
+  Request to overwrite existing directory (e.g. deploy existing build or change PR visibility when
+  the destination directory does already exist).
+
+- **413 (Payload Too Large)**:
+  Payload larger than size specified in `AIO_UPLOAD_MAX_SIZE`.
+
+
+## `https://*.ngbuilds.io/*`
+
+- **404 (Not Found)**:
+  Request not matched by the above rules.
+
+- **500 (Internal Server Error)**:
+  Error while processing a request matched by the above rules.