Confirm accuracy of IPNS docs page, and expand it

[ericronne]

This issue is part of Epic 3B: Fixes from legacy issue queue.

Is the info on the IPNS concept docs page still accurate? Double check that, and expand the page to make it a richer primer.

Additional content fodder …

• IPNS specs
• IPFS camp deep dives on fast ipns, link rot, and rotating keys
• The definitive guide to publishing content on the decentralized web
• What is InterPlanetary Naming System(IPNS)?

[ericronne]

(expansion would fall outside of hotfix territory, so that can be split out into a separate issue, if need be)

[jessicaschilling]

Closing due to overlap with #90. Adding notes from this issue to that issue.