Add base animation documentation

What does this MR do?

• This is a part of gitlab-org&10429. This MR adds a motion section with general information about interface animations.
• Renames Motion to Animation. There are a few reasons why:
  • To distinctly differentiate Brand Motion from Interface Animations.
  • motion is strongly associated with motion design, which creates videos, promotional materials, and other non-interface elements.
  • While motion encompasses a moving process, animation mostly refers to transitioning from one state to another.

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.

Links

gitlab-org/gitlab#430246 (closed) gitlab-org/gitlab#424397 (closed)