docs: document added rules

- the existence of the `kicker.json` file

- the existence of the `README.md` file

- for each variable in the `kicker.json` file (main template and variants):

    - emtpy default values should be omitted

    - mapped input is declared in the related template with same name, type, default, options and description (WARNING only)

    - the variable is declared in the related template with expected input mapping (`$[[ inputs.input-name ]]`)

    - the input/variable is documented in the README with same default (WARNING only)

    - for variables corresponding to container images:

        - [explicit image registry](https://to-be-continuous.gitlab.io/doc/dev/architecture/#explicit-official-image-registry) is specified

        - uses [latest tag by default](https://to-be-continuous.gitlab.io/doc/dev/architecture/#latest-version-by-default)

- for each job defined in GitLab templates:

    - the job starts with the [template (job) prefix](https://to-be-continuous.gitlab.io/doc/dev/guidelines/#prefix-job-names)

    - each report file complies to the [naming conventions](https://to-be-continuous.gitlab.io/doc/dev/architecture/#report-files-naming-convention)

If any non-WARNING rule fails, `tbc-check` fails with exit code 127.