Documentation of some sensible meta configuration options

1. Add the two missing "download on demand (DoD)" repositories:
    - 3.2.0.12_i486 (i586)  sailfishos:3.2.0.12/latest_i486
    - 3.2.0.12_armv7hl (armv8el)  sailfishos:3.2.0.12/latest_armv7hl
    - As a reference see https://github.com/sailfishos-chum/main/issues/76

2. Add the two missing "download on demand (DoD)" testing repositories:
    - sailfish_testing_i486 (i586)  sailfishos:latest/latest_i486
    - sailfish_testing_aarch64 (aarch64)  sailfishos:latest/latest_aarch64
    - As a reference see https://github.com/sailfishos-chum/main/issues/76

3. Document the global sections <publish>, <build>, <debuginfo> and <useforbuild>, which can be set to <enable/>, <disable/> (both global), plus e.g., <enable repository="3.2.0.12_armv7hl"/>, <disable arch="aarch64"/>, <enable arch="i586" repository="3.1.0.12_armv7hl"/> and combinations of these.

4. https://build.sailfishos.org/repositories/nemo:testing:hw:common goes back to:
    - sailfishos_2.0.1.11 (armv8el)  sailfishos:2.0.1.11/latest_armv7hl
    - sailfishos_4.0.1.48_aarch64 (aarch64)  sailfishos:4.0.1.48/latest_aarch64
    - Nothing, never (i586) i486
    - In addition to the first entry also:
      sailfishos_4.0.1.48_armv7hl (armv8el)  sailfishos:4.0.1.48/latest_armv7hl
    - As a reference see https://build.sailfishos.org/project/meta/sailfishos:chum