stone: Extend the documentation about headers

[livingsilver94]

This is a small PR that tries to improve the documentation about the global header of a stone.

I plan to further document the Stone format by adding pages about the payloads, but for now I'll stop with the header.
And since I can't help adding unsolicited edits, I thought that "Prelude" is a better name for the global header. In fact, there are three kinds of header in a stone:

• The global one, version-agnostic
• The data section of the agnostic header, version-dependent
• The sub-header of each payload

I think there are too many name clashes, hence my choice to introduce the name "Prelude" to uniquely identify the global header, version-agnostic.
I'm still not sure if we should call the version-dependent header e.g. "V1 prelude" or not.

The name "Prelude" comes from Rust's prelude.

[livingsilver94]

We have added Mermaid support in this repo. I'm setting the Draft mode, I may choose to use Mermaid.