Migrate spec import to new location docs/spec

[sebastianbenz]

Once ampproject/amphtml#34160 has landed. We probably should import all docs from master to avoid syncing the change with the next amp release.

[patrickkettner]

@sebastianbenz just to be sure - by master do you mean main?

[sebastianbenz]

Yes, but the parameter is still named master in our code. BTW, we're already defaulting to main for the spec import, so we should be fine: https://github.com/ampproject/amp.dev/blob/future/platform/lib/pipeline/specImporter.js#L51