document signal detector functions in the knowledgebase

[zachll]

Problem: The PFKB does not specify any details about signal detector functions that are used in the pipeline

Proposed solution:

• create a top-level motivating information detectors folder in the PFKB
• create a readme file for each detector group
• for trends, describe concepts that are consistent with PSDO definitions
• Add details about monotonicity, duration, and positive/negative trend values for the team to review
• Describe annotations as they are currently implemented, and include links to code in the PFP repo
• Consider adding existing description for related vignettes

[zachll]

Documentation in PFKB can follow work in #313 in the pipeline repository:https://github.com/orgs/Display-Lab/projects/5/views/1?pane=issue&itemId=55279476

[faridseifi]

Peter will take a first pass at documenting the Achievement and Loss detectors.