[tooling] Add make targets to generate requirements.txt

[BenjaminPelletier]

Currently, discovering how to handle changes to requirements can be tricky -- someone who has not read the repository intro closely would likely go to requirements.txt and see:

# This file is autogenerated by pip-compile with Python 3.12
# by the following command:
#
#    pip-compile --generate-hashes --output-file=requirements.txt requirements.in

And, when following these instructions, a user must discover that they need their own Python environment set up in an appropriate way, perhaps with a specific Python version, in order to run this command successfully if they did not discover make update-pinned-dependencies.

This PR attempts to standardize requirements specification by:

1. Adding a make target to generate requirements.txt from requirements.in using the standard interuss/monitoring image
2. Adding a make target to upgrade dependencies
3. Documenting these make targets in requirements.in
4. Modifying requirements.txt in the make recipe to refer users to requirements.in
5. Updating documentation to use make requirements.txt
6. Removing separate Docker image so that requirements.txt is built in the same environment it will be controlling

To do this, one additional dependency (pip-tools) is added to the monitoring requirements, and the changes to requirements.txt in this PR are generated by running the new make requirements.txt command after bootstrapping the image build beforehand.

[mickmis]

Nice!