First draft of Docs on Backup

[ttmc]

• A first draft of the section "Backing Up & Restoring Data"
• Moved the "Nodes, Clusters & Federations" (terminology) section from under "Topic Guides" to under "BigchainDB Clusters & Federations"
• Split the section "Set Up and Run a Federation" into separate sections under "BigchainDB Clusters & Federations", because it was going to be too long once all the documentation-to-come came. One of those new separate sections is the new backup section.

[codecov-io]

Current coverage is 89.57%

Merging #406 into master will not change coverage

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[r-marques]

👍

[ttmc]

I still want to add some notes about:

• Using RAID or similar on each node so that if one of the hard drives fails, the node keeps working as if nothing happened, and the failed hard drive can be replaced.
• Using RethinkDB's existing replication and server tags capabilities to create a set of dedicated "backup-only nodes" --- which would be functionally very similar to continuous backup.

so maybe this won't get merged until after the upcoming version-release.

[ttmc]

@r-marques could you look at the two short subsections I added in commit 877937c ? If you're okay with what I wrote there, then I'll merge this pull request.

[r-marques]

👍