Generation of readthedocs with mkdocs does not work anymore

[averbraeck]

Changes in configuration for readthedocs.io results in a failure when generating documentation.

[averbraeck]

For readthedocs, a new file called .readthedocs.yaml in the root directory of the project is needed, with the following content:

# Read the Docs configuration file for MkDocs projects
# 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"

mkdocs:
  configuration: mkdocs.yml

# Optionally declare the Python requirements required to build your docs
python:
  install:
  - requirements: docs/requirements.txt

[averbraeck]

The file requirements.txt has been moved to the docs folder to minimize the number of files in the root folder.

[averbraeck]

After this change, documentation is generated again on RTD.