Generation for readthedocs needs to be updated

[averbraeck]

Currently, the generation of documentation at readthedocs fails. The site has updated and asks for a different file structure than what we currently offer. As a result, we get:

[averbraeck]

A new config file named .readthedocs.yaml needs to be added in the root of the project.

See https://docs.readthedocs.io/en/stable/config-file/index.html for more details.

[averbraeck]

The mkdocs.yml file can be moved to the docs folder to keep the root folder as clean as possible. The same holds for the requirements.txt file.

The .readthedocs.yaml file then looks as follows:

# 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: docs/mkdocs.yml

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

[averbraeck]

The readthedocs files are generated again.