spec: Migrate the specification into a plain text document

[Richard Ulmer]

I find it inconvenient, that the specification is a Google Docs document. I would perefer the specification to be a simple text document, version controlled with git. Those are the drawback, that I see with Google Docs:

• Proposing changes is less clear and less convenient than a pull request would be.
• I cannot see the "true history" of the document and must rely on the "Changes" section.
• The PRIMARY selection ("middle-mouse-button-clipboard") does not work. This is merely an annoyance, but one I encountered repeatedly.
• Google Docs is slow (yes, this is a real complaint - on OpenBSD Firefox is quite slow on JavaScript heavy webpages).
• I cannot grep through the document.
• Following links seems somewhat cumbersome to me.

I would suggest creating a separate repository for the specification, because it should be independent of the Go implementation. In case you want to do this, I have already prepared a repo for you to fork: https://github.com/codesoap/age-spec. The redirect of https://age-encryption.org/v1 would need to be changed afterwards.