Rubberise main layout

[mejedi]

Currently, the main column width is fixed (like 800px).

It would be nice if it could expand, taking available space (with a max-width to prevent from growing too wide).

Related: #9

Depends on: update header / footer layout; TOC grey background is implemented using a background image in body.

[bigbes]

We needed to have rubberized layout for documentation.

We need 1220px width max (previously 800-900px), where:

• 300px for TOC (Tree of Contents)
• 20px for delimiter (for now, it won't have any functional meaning; later it'll help us hide/show TOC on demand)
• 900px(max-width) --down to--> 600px(min-width) for Content

Before:

After (for now, ignore "hideable" in TOC, it's a future feature):