While not building documentation that used to work just fine before recently a rebuild produced

Error

Your documentation did not generate an index.html at its root directory. This is required for documentation serving at the root URL for this version.

The YAML config file I used is fairly standard

# .readthedocs.yaml
# Read the Docs configuration file
# See https://docs.readthedocs.io/en/stable/config-file/v2.html for details

# Required
version: 2

# Set the version of Python and other tools you might need
build:
  os: ubuntu-22.04
  tools:
    python: "3.12"

# Build documentation in the docs/api directory with Sphinx
sphinx:
  builder: html
  configuration: docs/api/conf.py
  # TODO: needs dropping of additional customizations
  fail_on_warning: false

# Build docs in additional formats such as PDF and ePub
formats: all

# Specify dependencies to enable reproducible builds:
# https://docs.readthedocs.io/en/stable/guides/reproducible-builds.html
python:
  install:
    - requirements: packaging/pip_requirements_minimal.txt
    - method: pip
      path: ./packaging/

Does anyone happen to know that does RTD want from me?