"Overview" in nomad GUI homepage

It would be informative to have an overview with basic statistics, supported codes, used normalizer, etc. It should show

• repo statistics: number of entries, datasets
• archive statistics: number of quantities, number of sccs, TBs
• raw statistics (?): TBs
• parsers: list with URL to code
• normalizer: list with description
• metainfo quantity statistics: number in public, number overall
• nomad version