Add job to generate dictionary

What does this MR do?

Add a CI job to generate automatically the Metrics Dictionary .md file

This uses a rake task to generate an md file

Each time we change a YAML file we should update the dictionary, this could help having the dictionary up to date.

Example metric file https://gitlab.com/gitlab-org/gitlab/-/blob/master/config/metrics/counts_all/20210216181249_feature_flags.yml

Dictionary md file https://gitlab.com/gitlab-org/gitlab/-/blob/master/doc/development/usage_ping/dictionary.md

Generated output https://docs.gitlab.com/ee/development/usage_ping/dictionary.html

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team