Add vault chapter to account docs

We may want to add a vault chapter to the account docs. This could explain:
- the underlying data structure of the vault (sparse merkle tree)
- how assets are added and removed
- its capacity for storing a large number of assets.

If this turns out to not be enough info to warrant its own chapter, we could also inline it into the account overview page.

If we add it as a separate chapter, we should try to order the chapters as mentioned here:
https://github.com/0xMiden/miden-base/pull/1792#discussion_r2301869905