docs(website): align docs sidebar and remove rail footer links

[cpcloud]

What

Adjust the docs sidebar so it aligns with the main content column on desktop, no longer acts as its own scroll container, and remove the Blog/GitHub links from the bottom of the left rail.

Context

• The current docs sidebar is sticky with height: 100vh and overflow-y: auto, which makes it scroll independently from the page.
• The left rail also includes extra Blog/GitHub links at the bottom that are no longer wanted.

Reproduce

1. Open a docs page on desktop.
2. Compare the left rail against the main content column; the rail feels offset and behaves as a separate scroll area.
3. Scroll over the sidebar and notice it scrolls independently.
4. See the Blog/GitHub links at the bottom of the rail.

[cpcloud]

This was fixed by #883.