Remove `@` from keywords

[iherman]

Removed the @ sign from the JSON-LD keywords @id, @type, @language, and @value, as agreed on a call for #312. (Not closing that issue, because there is still editorial work do to related to that issue. This meant

• updated the JSON LD context file
• updated the main document
• updated all the manifest examples
• updated all the diagrams
• updated the wp.js script
• updated the JSON schema files (@HadrienGardeur you should check that; e.g., the result is that the type term is now both a json schema keyword and a term in the data. I hope json schema can cope with that)

I have also

• made some minor editorial cleanups
• per request of @TzviyaSiegman I have removed the note on relating to the rel='canonical'.

Fix #324

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[iherman]

Based on the discussion on #312 this PR does what we have decided. However, we may want to wait a bit to see if the changes in the JSON schema are fine or not (@HadrienGardeur). Then again, if there is an issue, that can be taken care of later...

[iherman]

@HadrienGardeur thanks, I missed this one. I have changed that, too.

Merging and closing now.