creating an install procedure for lektor_markdown_image_attrs

lavamind has been trying to get lektor_markdown_image_attrs to actually play nicely with our packages directory !73 (merged). because the plugin uses the hatch build system, it doesn't have a setup.py file and pip won't install it. the solution is to build a source distribution and to have pip install that.

the problem is that lektor plugins reinstall doesn't install the source distribution by default. running pip install package/* works great, but it's not currently part of our CI flow, or our documentation.

which brings us to the point of this ticket: agreeing on a way to install this new package since our current method doesn't work

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we've decided on using a requirements.txt file in each repository to manage dependencies. the next steps are:

1. move lektor-requirements.txt from tpo/tpa/ci-templates to tpo/web/lego
2. update lego everywhere
3. set ci builds to install lego/lektor-requirements.txt and requirements.txt (if it exists)
4. add a requirements.txt file to any lektor repo that uses additional packages
5. add lektor_markdown_image_attrs as a dependency in lego/lektor-requirements.txt