Remake handbook flowchart in an editable format

https://github.com/astropy/astropy-mentoring/blob/main/mentoring_process_flowchart.png

Maybe a svg or some other markdown based thing.

Start by opening mentoring_process_flowchart.png and reviewing the handbook materials in the repository. Determine an editable representation that fits the project, then replace or supplement the image with that source. Done means the flowchart remains readable and can be edited without recreating the image manually.

Written by the indexing model from the issue text.