docs(V5): document V5 API migration and add V5 signal documentation

[JavierMtzRdz]

Summary:

• Adds a new V4 to V5 Migration Guide and updates V5 Sources and Signals with a V4 to V5 coverage table.
• Adds "Heads up" deprecation callouts to the handful of active V4 sources without a V5 equivalent (quidel, nchs-mortality, fluview/flusurv/fluview_clinical/fluview_meta) pointing at the migration guide, and to the sources that already have one (nssp, nhsn, doctor-visits to claims_data_outpatient, hospital-admissions to claims_data_inpatient, covidcast_meta to the V5 metadata endpoints) pointing users straight at the replacement
• Inactive/no-longer-updated sources are intentionally left untouched since they're staying on V4 indefinitely.

Prerequisites:

• Unless it is a documentation hotfix it should be merged against the dev branch
• Branch is up-to-date with the branch to be merged with, i.e. dev
• Build is successful
• Code is cleaned up and formatted

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud