docs: Add Realm ABI Specification

[mvertes]

The Realm ABI Specification formally describes the protocol and encoding of interactions between realms and external clients, not necessarily written in Go.

It can be seen as an extension of the ABCI which covers the protocol between blockchain nodes and clients, but without details on their practical applications, such as realms.

This PR addresses [#1842].

This is an initial draft commit, work in progress.

Contributors' checklist...

• [*] Added new tests, or not needed, or not feasible
• [*] Provided an example (e.g. screenshot) to aid review or the PR is self-explanatory
• [*] Updated the official documentation or not needed
• [*] No breaking changes were made, or a BREAKING CHANGE: xxx message was included in the description
• [*] Added references to related issues and PRs
• [ ] Provided any useful hints for running manual tests
• [ ] Added new benchmarks to generated graphs, if any. More info here.

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All modified and coverable lines are covered by tests :white_check_mark:

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