content: add v3.4.0 documentation

[gyuho]

/cc @philips @hexfusion @spzala @xiang90 @jpbetz @jingyih @wenjiaswe

[philips]

This doesn't look right.

New page:
https://deploy-preview-42--etcd.netlify.com/docs/v3.4.0/

Old page:
https://etcd.io/docs/v3.3.12/

Notice the documentation sections is empty and the categories too.

Do not merge this as-is.

[gyuho]

Oh, let me fix

[lucperkins]

@gyuho Several months ago I attempted to add the appropriate structure for the Documentation folder in the etcd repo: etcd-io/etcd#10615. At this point, it's probably too late to merge those changes painlessly, but that PR can act as a guide for how to restructure here. Most notably, see how the _index.md files indicate sections.

[gyuho]

@lucperkins @philips Now looks good https://deploy-preview-42--etcd.netlify.com/docs/v3.4.0/