feat(LoadMore): Add component guidelines

What does this MR do?

We want to add a new pattern, that will eventually replace infinite scroll, called Load more. I don't think this is a component, but I didn't find it to be a region either, so until we consider creating Patterns in Pajamas, I think this belongs in components. This was created based on Infinite scroll > Accessibility Audit (gitlab-org/gitlab-ui#1196)

Infinite scroll isn't an accessible pattern and should be replaced with a "load more" pattern. As-is, there isn't really anything that can be tested in GitLab UI since the example list of items doesn't have anything focusable, but even so there's no indication of the number of results or expected/actual behavior.

Related to #1390 (closed)

Does this MR meet the acceptance criteria?

• The MR title and commit messages meet the Pajamas commit conventions.
• The “What does this MR do?” section in the MR description is filled out, explaining the reasons for and scope of the proposed changes, per “Say why not just what”.
  • For example, if the MR is focused on usage guidelines, addressing accessibility challenges could be added in a separate MR.
• Relevant label(s) are applied to the MR.
• The MR is added to a milestone.
• If creating a new component page from scratch, it follows the page template structure.
• Content follows the Pajamas voice and tone guidelines, falling back on the GitLab Documentation Style Guide when needed.
• Related pages are cross-linked, where helpful. Component pages have related components and patterns defined in their Markdown front matter.
• If embedding a Figma file, it follows the Figma embed guide.
• Review requested from any GitLab designer or directly from a maintainer or trainee maintainer.

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