Revises emoji section of Markdown page

What does this MR do?

As a prototype for other potential changes to this page, this commit changes the emoji display from sequential to tab-based.

More importantly, it also adds a section about GitLab custom emoji, because this is an undocumented feature that most folks don't realize exists because it's not in the emoji picker yet.

It also adds some extra CSS formatting to the images to make them display better.

Current version

• Padding values make the emoji smaller than intended.
• Markdown example needs hard wraps.
• Rendered markdown uses hard-coded images, but they lack alt / title attributes so it's tough to tell what emoji are in use

Click to expand

Proposed changes

• Cleaned up image presentation
• Placed markdown + rendered output in tabs
• Hard wraps in code section
• Oh yeah, GitLab custom emoji

See:

• https://main-ee-115595.docs.gitlab-review.app/ee/user/markdown.html#emojis
• New, shiny: https://main-ee-115595.docs.gitlab-review.app/ee/user/markdown.html#gitlab-custom-emojis

Related issues

• Related to REQUEST YOUR EMOJI HERE (custom_emoji/custom_emoji#1)
• Related to Add custom emoji to award emoji picker (#333095 - closed)
• Related to Create custom emoji on instance-level (#23717)

The custom emoji need to be updated, so here are even more related issues!

• Related to https://gitlab.com/gitlab-com/marketing/corporate_marketing/corporate-marketing/-/issues/6419+ where the values emoji are being worked on
• Related to Update values emoji after they're revised (custom_emoji/custom_emoji#3 - closed) to update the copies of the images in the emoji repo afterward

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.