Add files for readthedocs

.readthedocs-custom-steps.yml

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+steps:
+- pydoc-markdown --build --site-dir $SITE_DIR

.readthedocs-requirements.txt

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+readthedocs-custom-steps

.readthedocs.yml

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+version: 2
+mkdocs: {}  # tell readthedocs to use mkdocs
+python:
+  version: 3.7
+  install:
+  - method: pip
+    path: .
+  - requirements: .readthedocs-requirements.txt