Add explanatory info/context to IPLD Explorer

**This issue is one of the prioritized recommendations in the [Q1 2020 IPFS Ecosystem Audit](https://github.com/ipfs/community/tree/master/ecosystem-audit) for execution in Q2/Q3 2020. Tagging it with `$auditrecommendation` means it's searchable! See [all tagged issues](https://github.com/ipfs/docs/issues?q=is%3Aissue+%22%24auditrecommendation%22+) in this repo.**

IPLD Explorer is awesome once you know what all of it means ... but that's making a massive assumption. We're missing out on a large learning opportunity.

Add explanatory info and context to the site, so someone inexperienced in IPFS could use this as an entry point to learning more in general. This could include ...
- Inroductory content provided in a non-intrusive way, so it could ignored by those in the know
- Hover links with extensive cross-linking to docs, ProtoSchool, etc

Also see https://github.com/ipfs-shipyard/ipfs-companion/issues/610 for additional brainstorming on better integrating IPLD Explorer.