Documentation: add Nix installation instructions

[dannywillems]

On top of https://github.com/o1-labs/openmina/pull/1234.

The build instructions should be included in the getting started section, see https://github.com/o1-labs/openmina/blob/dw/docs-docusaurus/website/docs/developers/getting-started.mdx.
The instructions should be built in the CI, as it is here for Ubuntu.

The CI jobs are run on nightly or on demand.
act -j [job_name] can be used locally to replicate CI behavior.

[dannywillems]

Fixed in https://github.com/o1-labs/openmina/pull/1241