Documentation improvement - Part 2

[unmarshall]

How to categorize this issue?

/area documentation
/kind enhancement

What would you like to be added:
#813 got closed prematurely. Most of the documentation changes have been done but some are still pending. This issue captures the pending items.

• Use mkdocs to generate github pages for documentation (Introducing mkdocs generated github pages for etcd-druid documentation #909)
• Add docs/usage/copy-backups.md
• Add a documentation to explain auto-single-member restoration under docs/concepts
• Add doc to explain snapshot compaction under docs/concepts
• Add API reference docs and include them in mkdocs

Why is this needed:
To improve ease of consumption.