Resolve pinned sidebar render glitch

What does this MR do?

It prevents the sidebar from animating on page switch when it is in pinned / closed state. It adds a class to header and page after the other sidebar classes are added marking it as "rendered". Further it sets css transition only for those elements that are "rendered".

Are there points in the code the reviewer needs to double check?

No

Why was this MR needed?

Because the sidebar showed some odd behavior when in pinned / closed state.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #23988 (closed)