Document SFMS/PMtiles naming scheme

**Describe the task**
The existing naming scheme for SFMS is unobvious since dates imply different things in the full path to a file (issue date, for date). There are comments that describe the scheme in the code but are a bit buried. Let's document the scheme in a markdown file linked to from the main README.

**Acceptance Criteria**
- [ ] The naming scheme is documented in a markdown file in the repo
- [ ] It has been reviewed by all developers

**Additional context**
- Determining actual or forecast for file name: https://github.com/bcgov/wps/blob/bc1825ee0037042f2707a10ff069e1f1b628abac/api/app/auto_spatial_advisory/sfms.py#L23
- Or here

Start with the naming comments in api/app/auto_spatial_advisory/sfms.py at the referenced location, then inspect the main README for the appropriate link. Document the SFMS/PMtiles naming scheme in a repository markdown file, link it from the README, and have all developers review it.

Written by the indexing model from the issue text.