Add Containers reference page and link from reference index

[Mahajan-Sachin]

Closes #2032

Adds a dedicated reference page for containers under docs/reference/containers.

Includes:

• Container
• Vertical
• Horizontal
• Default behavior
• Default CSS

Also fixes navigation link so the Containers reference page loads correctly.

[willmcgugan]

Thanks. This is needed, but I would want to do a more comprehensive reference with diagrams and examples.

if you want to go to that effort, I wold be happy to assist. But I can't merge this as is.

[Mahajan-Sachin]

Thanks for the feedback!

I'd be happy to expand this into a more comprehensive reference with diagrams and practical examples.

To make sure I align with your expectations, would you prefer the focus to be on:

• Real-world usage examples?
• Visual layout diagrams explaining container behavior?
• Comparisons between container types?
• Or a combination of these?

Happy to iterate in the direction you think would be most valuable.