Add a lock file for requirements-docs.txt

[jonathan343]

This PR uses the pip-compile command from pip-tools to generate a lock file for requirments-docs.txt. This was generated in Python 3.8 virtual environment.

NOTE: The following urllib3 requirements were added to requirements-docs.txt to force the same version constraints as botocore when generating the requirements-docs-lock.txt:

urllib3>=1.25.4,<1.27 ; python_version < "3.10"
urllib3>=1.25.4,!=2.2.0,<3 ; python_version >= "3.10"

This specifically becomes an issue when using Python 3.8 since Sphinx has a transitive dependency on the latest version of urllib3 and botocore will require urllib3>=1.25.4,<1.27.