Chore: Remove configuration for Read the Docs (#595)

https://responder.kennethreitz.org/ is here to stay.
This patch concludes the decision in GH-564.

.readthedocs.yml

@@ -1,33 +0,0 @@
-# .readthedocs.yml
-# Read the Docs configuration file
-
-# Details
-# - https://docs.readthedocs.io/en/stable/config-file/v2.html
-
-# Required
-version: 2
-
-build:
-  os: "ubuntu-24.04"
-  tools:
-    python: "3.12"
-
-python:
-  install:
-      - method: pip
-        path: .
-        extra_requirements:
-          - docs
-
-sphinx:
-  configuration: docs/source/conf.py
-
-  # Use standard HTML builder.
-  builder: html
-
-  # Fail on all warnings to avoid broken references.
-  fail_on_warning: true
-
-# Optionally build your docs in additional formats such as PDF
-#formats:
-#  - pdf