doc: documented $PREF_TAG_PATTERN variable

* `GITLAB_TOKEN`: a [personal access token](https://docs.gitlab.com/ee/user/profile/personal_access_tokens.html) with scopes `api,read_repository`

  and at least `Developer` role on all groups & projects to crawl (not required if only `public` groups and projects),

* `PREF_TAG_PATTERN`: preferred tag pattern.

* `KICKER_RESOURCE_GROUPS`: JSON configuration of GitLab groups to crawl.

### Details about `PREF_TAG_PATTERN`

The algorithm to determine which template latest version to include in Kicker is the following:

1. get all published (Git) tags, 

2. sort them (using [Version sort](https://www.gnu.org/software/coreutils/manual/html_node/Version-sort-overview.html)),

3. among this list, filter all tags matching a _preferred pattern_ (regex)

    * one or more matches: return the latest one from the filtered list

    * none matches (fallback): simply pick the latest one from the unfiltered list

The `PREF_TAG_PATTERN` variable allows to define the preferred version pattern to be used as the latest version to include in Kicker.

By default it is set as `^v?[0-9]+\.[0-9]+$`. In other words, Kicker will propose the latest published 

minor version alias (ex `2.4`), when present.

If your want another behavior in your self-managed GitLab, simply override this variable in your local copy 

of the `to-be-continuous/doc` project. Example values:

* full semver version (incuding fix part): `^v?[0-9]+\.[0-9]+\.[0-9]+$`

* simply the latest tag (whichever the format): `.*`

### Details about `KICKER_RESOURCE_GROUPS`

Here is an example of `KICKER_RESOURCE_GROUPS` content:

```json

    log_info "       [--json-out <output JSON file>]"

    log_info "       [--doc-out <README output dir>]"

    log_info "       [--groups <JSON encoded GitLab groups to crawl>]"

    log_info "       [--tag-pattern <preferred tag pattern (regex)>]"

    echo

    log_info "Groups to crawl shall be formatted as follows:"

    log_info "[