Clarify Indigo's website contribution and building
Clarification and motivation
Indigo's website does not explain how to open issues or merge requests for it.
Additionally, its directory's README does not specify well how to build it or check the result of changes.
Acceptance criteria
Indigo's website index
page has a paragraph about contributing.
Indigo's website source's README both the methods that can be used to build it (nix and mkdocs).