Add discussion of `params.json` and defaults to indicator manual

[nmdefries]

Indicator manual

This section should discuss:

• the local and ansible/production templates
• params sections
• standard params that MUST be supported
• optional standard params
• how to choose defaults
  • defaults MUST allow production runs to be run every day without updating the params file

We also need to actually design a standard for params.json so that the indicators work more similarly.

[nmdefries]

Patch params section should probably always include a flag turning the patching functionality on/off.

[aysim319]

We also don't have any validation for the params.json; Also would be nice to mention read_params() function; currently the function just json.loads from the file;
Context:
https://github.com/cmu-delphi/covidcast-indicators/pull/2000/files#r1709806702