Add known/standard metadata keys to documentation

[jphastings]

It'd be great if the metadata section of the docs was expanded to list all the known/standard keys and their intended purpose & format.

Right now it's not clear if we should make our own if they're unlisted (eg. servings isn't mentioned there, but it appears to be considered standard from other pages in the docs).

What I'm assuming the list would be so far:

Key	Purpose	Example value
source	Where the recipe came from. Usually a URL, can also be text (eg. a book title) with anything hidden from display (ie. for machine use) in angle brackets afterwards.	https://example.org/recipe, The Palomar Cookbook <urn:isbn:9781784720995>
time required	The preparation + cook time of the recipe. Various formats can be parsed, if in doubt use HhMm format to avoid plurals and locales.	45 minutes, 1 hour 30 minutes,1h30m
servings	Indicates how many people the recipe is for Used for scaling quantities. Leading number is used for scaling, anything else is ignored but shown as units.	2,15 cups worth
course	??? I'm not sure! This looks like it'd hold a category name, rather than a course, especially when multilingual stuff comes into play?	dinner
locale	The locale of the recipe. Used for spelling/grammar during edits, and for pluralisation of amounts. Uses ISO 639 language code, then optionally an underscore and the ISO 3166 alpha2 "country code" for dialect variants	es_VE, en_GB, fr

I'd love to have clarity on these as I'd like to know where I can put information in software I'm writing such that other cooklang tools can interpret it correctly.

For example, if I'm turning a recipe from a physical book into a cooklang recipe I'd like to be able to store the ISBN and the page number(s). I'd be tempted to put that like this, but I'd want to know that (a) the name of the source would be used and (b) the URN would be hidden (possibly linked to for clients that support ISBN URNs) (edit: it looks like this is assumed by the cooklang extensions, can I consider them standard?)

source: The Palomar Cookbook <urn:isbn:9781784720995#pages=42-43>

I'd like to propose the following keys too:

Key	Purpose	Example value
cook time	The time it take to just cook the recipe. Formatted as time required field	4 hours, 4h
prep time	The time it take to just prep the recipe. Formatted as time required field	15 minutes,1h30m

I'm new here, but I'll be happy to make a PR if this makes sense.

[jphastings]

I hadn't spotted that this has been partially done with the cooklang rs extensions — it'd be great if this was referenced too!

[dubadub]

Relevant proposal for spec change #105.

[dubadub]

This is now part of the main specification, closing the discussion.