Document updated main/nightly component configuration for apt-test

[eloquence]

https://github.com/freedomofpress/securedrop-workstation/pull/970 added main to the list of components for apt-test.freedom.press in a dev environment. This changes the behavior and will install more recent packages from main when they're available (as is the case of this writing with RC1 of https://apt-test.freedom.press/pool/main/s/securedrop-client/securedrop-client_0.11.0~rc1%2Bbookworm_all.deb). Documenting this to avoid confusion

Checklist (Optional)

• [x] Doc linting (make docs-lint) passed locally
• [ ] Doc link linting (make docs-linkcheck) passed
• [x] You have previewed (make docs) docs at http://localhost:8000

[eloquence]

Closing in favor of https://github.com/freedomofpress/securedrop-workstation/issues/1106