Add documentation for Miden crypto

# We want documentation for Miden crypto

As described in https://github.com/0xPolygonMiden/miden-base/issues/472, we want documentation for the Miden crypto. The documentation source files should be located in the crypto repo. Then, developers can update the docs using the same PR as the code.

## Target group for the Miden crypto docs

We have different target groups for our documentation:

1. People who want to develop on top of Polygon Miden - e.g., write new smart contracts (accounts, notes), build new applications, etc. For them, they should be able to understand the underlying primitives and trade-offs
2. People who want to contribute to Polygon Miden - e.g., those who may be interested in making proposals to improve the cryptography or contribute to the development etc.
3. People who want to use Polygon Miden. For now, this is mostly people who may want to play with it - but in the future, it can be people who want to participate in running the network.

For now, the order of importance is the above.

## Proposed structure

- **let's discuss / settle on a structure in this issue**

## Mkdocs
We are going to use Mkdocs and a [plugin](https://github.com/jdoiro3/mkdocs-multirepo-plugin) to build the Miden unified docs in [docs.polygon.technology](https://docs.polygon.technology/miden/).

So please build the docs with [mkdocs](https://squidfunk.github.io/mkdocs-material/) material already in mind.