Add few WIP pages

[aguschin]

Improve these WIP pages and add them to the sidebar:

{
  "slug": "datasets",
  "label": "WIP Working with datasets",
  "source": "user-guide/datasets.md"
},
{
  "slug": "remote-repos",
  "label": "WIP Working with repositories and remote objects",
  "source": "user-guide/remote-repos.md"
},
{
  "slug": "mlem-object",
  "label": "MlemObject API",
  "source": "api-reference/mlem-object.md"
},

[jorgeorpinel]

Yep we can start by reversing 9b60b8c