Backport of "Adjustments to Wiki experience" to 19.4

What does this MR do and why?

Backport of !256748 (merged) to 19-4-stable-ee. It cherry-picks the squash commit 5064917d with -x. The cherry-pick applied cleanly and the patch is identical to the one merged to master.

It fixes these wiki layout defects:

• The sticky header offset is wrong and doesn't follow the sidebar width when the sidebar is resized or collapsed.
• The padding inside the panel is wrong.
• The sidebar width is now stored in the --wiki-sidebar-width CSS variable, so the content and the sticky header follow the sidebar when it's resized.

The wider content widths (fixed layout 768px → 940px, fluid layout 992px → 1600px) come along with these fixes.

This backport was requested for a customer. The EM agreed to treat it as a typebug bugux backport in !256748 (comment 3998026888). Product sign-off from @gweaver is still pending.

Related backports: !261745 (merged) (19.3), !261746 (19.2).

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• This MR is backporting a bug fix, documentation update, or spec fix, previously merged in the default branch.
• The MR that fixed the bug on the default branch has been deployed to GitLab.com (not applicable for documentation or spec changes).
• The MR title is descriptive (e.g. "Backport of 'title of default branch MR'"). This is important, since the title will be copied to the patch blog post.
• Required labels have been applied to this merge request
  • severity label and bug subtype labels (if applicable)
  • If this MR fixes a bug that affects customers, the customer label has been applied.
• This MR has been approved by a maintainer (only one approval is required).
• Ensure the E2E Omnibus GitLab EE pipeline (triggered by thee2e:test-on-omnibus-ee job) has succeeded. If it has failed (or shows warnings), investigate the failures. If you determine the failures are unrelated, you may proceed. If you need assistance investigating, request help in the #s_developer_experience Slack channel to confirm the failures are unrelated to the merge request.

Note to the merge request author and maintainer

If you have questions about the patch release process, please:

• Refer to the patch release runbook for engineers and maintainers for guidance.
• Ask questions on the #releases Slack channel (internal only).
• Once the backport has been merged, the commit changes will be automatically deployed to a release environment that can be used for manual validation. See after merging runbook for details.