Write an article about the gas tree data structure and algorithms

[shamilsan]

• Source: https://github.com/gear-tech/gear-tech.github.io/blob/master/src/protocol/gas-tree.md
• Output: https://gear.rs/protocol/gas-tree.html

Need to describe in detail what our famous gas tree (or even gas forest) data structure looks like and what are the conditions of state transition. We can use mermaid for diagrams (it has been already integrated), and links to the chunks of source code would be much appreciated (use some tag for links not to become dead after some time).