specs: Specify node types

[rootulp]

Context

https://github.com/celestiaorg/celestia-specs was deprecated in favor of specs per repo. See celestiaorg/celestia-specs#212

Problem

celestia-node doesn't have a specs directory yet.

Proposal

When we create a specs/ directory in this repo, we may draw inspiration from node_types.md. Additionally we should explain under what conditions a celestia-node should halt. Motivated by celestiaorg/celestia-app#1436 (comment)

[rootulp]

Re-opening because celestia-node is getting it's own specs soon (#3186) so this issue became relevant again.

[Wondertan]

Changing this issue to track the node type specification from general "Consider creating specs"