Introduce draft for text binding template

[relu91]

I've started working on new documents for different payload binding templates. As suggested in #139 it might be easier to have a section that explains how to map data (conformant to a particular content type) to "Data Schema values". Informally we can think of "Data Schema values" as JavaScript values, but I would like to have them rigorously defined somewhere.

This PR is of course still heavily WIP, but I'd like to have feedback as soon as possible about the new document structure.

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[relu91]

I'm still grinding on this. I've updated a little bit the structure and text (also in the main document). TODOs:

• correctly cite our own terms (Thing Description, Data Schema etc.)
• Discuss the changes in the Payload Representation section

[egekorkan]

I think that it looks good overall. For changes to the main document, I think it is needed to refer to https://w3c.github.io/wot-thing-description/#sec-data-schema-vocabulary-definition
For changes to the text binding, the most "frustrating" part is that the word text is used as a non-technical word as well (a bit like Thing). So when you say text binding or similar, we should agree on what to use. Some suggestions could be always saying:

1. Text Plain (done once)
2. text/plain
3. Text with capital T and putting a note somewhere saying something like "The word Text when used with a capital T means the text/plain media type"

I see some typo or english mistakes but I will go over when the document is more stable. Also, Fujitsu had a Thing with this media type. One example I could find is here: https://github.com/w3c/wot-testing/blob/main/events/plugfest/2019-tpac-fukuoka/TDs/Fujitsu/local/Fujitsu-AgriDevice.jsonld#L61

@mryuichi could you share your experience with using this media type in TDs?