Use DB for Documentation meta storage

[everzet]

I'm not sure about usage of Filesystem for DocumentationRepository, but that's the start and we can always introduce more heavyweight DB-based repo. Also, as I'm planning to cache all documentation pages extensively, maybe that's not that big of a deal.

[jakzal]

Since all the doc pages can be fully cached I don't think it's worth it. At least not at this stage.

[stof]

But we will probably need to read the meta storage to set the cache headers (last-modified date). So this storage needs to be fast enough