Add section about custom GitLab values emoji

What does this MR do?

Revives the first commit from an old merge request (Revises emoji section of Markdown page (!115595 - merged)) which was originally about documenting the existence of custom GitLab values emoji in the Markdown page. (https://docs.gitlab.com/ee/user/markdown.html#emojis) We previously decided not to publish the list of custom emoji, because @vicbell was working on new renditions.

Related issues

So many!

• Related to https://gitlab.com/gitlab-com/marketing/corporate_marketing/corporate-marketing/-/issues/7774+
• Related to REQUEST YOUR EMOJI HERE (custom_emoji/custom_emoji#1)
• Related to https://gitlab.com/gitlab-com/marketing/corporate_marketing/corporate-marketing/-/issues/6419+
• Related to Update values emoji after they're revised (custom_emoji/custom_emoji#3 - closed)

Author's checklist

• Optional. Consider taking the GitLab Technical Writing Fundamentals course.
• Follow the:
  • Documentation process.
  • Documentation guidelines.
  • Style Guide.
• If you're adding or changing the main heading of the page (H1), ensure that the product tier badge is added.
• If you are a GitLab team member, request a review based on:
  • The documentation page's metadata.
  • The associated Technical Writer.

If you are a GitLab team member and only adding documentation, do not add any of the following labels:

• ~"frontend"
• ~"backend"
• ~"type::bug"
• ~"database"

These labels cause the MR to be added to code verification QA issues.

Reviewer's checklist

Documentation-related MRs should be reviewed by a Technical Writer for a non-blocking review, based on Documentation Guidelines and the Style Guide.

If you aren't sure which tech writer to ask, use roulette or ask in the #docs Slack channel.

• If the content requires it, ensure the information is reviewed by a subject matter expert.
• Technical writer review items:
  • Ensure docs metadata is present and up-to-date.
  • Ensure the appropriate labels are added to this MR.
  • Ensure a release milestone is set.
  • If relevant to this MR, ensure content topic type principles are in use, including:
    • The headings should be something you'd do a Google search for. Instead of Default behavior, say something like Default behavior when you close an issue.
    • The headings (other than the page title) should be active. Instead of Configuring GDK, say something like Configure GDK.
    • Any task steps should be written as a numbered list.
    • If the content still needs to be edited for topic types, you can create a follow-up issue with the docs-technical-debt label.
• Review by assigned maintainer, who can always request/require the reviews above. Maintainer's review can occur before or after a technical writer review.