Document `BasechainAddress` struct and related functions

[novusnota]

Introduced in #2035. It had some doc comments, but not the documentation itself.

• struct BasechainAddress (available since 1.6.0)
• emptyBasechainAddress (available since 1.6.0)
• newBasechainAddress (available since 1.6.0)
• contractBasechainAddress (available since 1.6.0)
• storeBasechainAddress (available since 1.6.0)
• add the new functions to CHANGELOG

[novusnota]

Add docs for #2097 too.

[novusnota]

And a new suggestion on the gas best practices page (#2342)