doc: fix broken links

Here are some advices about your **secrets** (variables marked with a :lock:):

1. Manage them as [project or group CI/CD variables](https://docs.gitlab.com/ee/ci/variables/#create-a-custom-variable-in-the-ui):

    * [**masked**](https://docs.gitlab.com/ee/ci/variables/#mask-a-custom-variable) to prevent them from being inadvertently

1. Manage them as [project or group CI/CD variables](https://docs.gitlab.com/ee/ci/variables/#add-a-cicd-variable-to-a-project):

    * [**masked**](https://docs.gitlab.com/ee/ci/variables/#mask-a-cicd-variable) to prevent them from being inadvertently

      displayed in your job logs,

    * [**protected**](https://docs.gitlab.com/ee/ci/variables/#protect-a-custom-variable) if you want to secure some secrets

    * [**protected**](https://docs.gitlab.com/ee/ci/variables/#protected-cicd-variables) if you want to secure some secrets

      you don't want everyone in the project to have access to (for instance production secrets).

2. In case a secret contains [characters that prevent it from being masked](https://docs.gitlab.com/ee/ci/variables/#masked-variable-requirements),

2. In case a secret contains [characters that prevent it from being masked](https://docs.gitlab.com/ee/ci/variables/#mask-a-cicd-variable),

  simply define its value as the [Base64](https://en.wikipedia.org/wiki/Base64) encoded value prefixed with `@b64@`:

  it will then be possible to mask it and the template will automatically decode it prior to using it.

3. Don't forget to escape special characters (ex: `$` -> `$$`).

  }

  function awkenvsubst() {

    awk '{while(match($0,"[$]{[^}]*}")) {var=substr($0,RSTART+2,RLENGTH -3);val=ENVIRON[var];gsub(/["\\]/,"\\\\&", val);gsub("\n", "\\n", val);gsub("\r", "\\r", val);gsub("[$]{"var"}",val)}}1'

    awk '{while(match($0,"[$%]{[^}]*}")) {var=substr($0,RSTART+2,RLENGTH-3);val=ENVIRON[var]; gsub(/["\\]/,"\\\\&",val); gsub("\n","\\n",val);gsub("\r","\\r",val); gsub("[$%]{"var"}",val)}}1'

  }

  function configure_registries_auth() {