docs: add doc for nydusd failover and hot upgrade

[loheagn]

Relevant Issue (if applicable)

Details

This patch adds doc for nydusd failover and hot upgrade features. The doc describes how to use these features and the design.

Types of changes

What types of changes does your PullRequest introduce? Put an x in all the boxes that apply:

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation Update (if none of the other choices apply)

Checklist

Go over all the following points, and put an x in all the boxes that apply.

• I have updated the documentation accordingly.
• I have added tests to cover my changes.

[imeoer]

cc @ccx1024cc