[DOC] better documentation for /_render/template API

[rursprung]

What do you want to do?

• Request a change to existing documentation
• Add new documentation
• Report a technical problem with the documentation
• Other

Tell us about your request.
the /_render/template API is barely documented, it is only mentioned in passing and not even in a chapter on its own:

documentation-website/_api-reference/search-template.md

Lines 213 to 218 in de9a7ca

	```json
	GET /_render/template
	POST /_render/template
	GET /_render/template/<id>
	POST /_render/template/<id>
	```

this should at the very least be a dedicated chapter within that page if not a page on its own (i don't know if you plan on having one page per API - which might be nice - or have them grouped together; if you create a dedicated page it should be linked on the current page so that it can be found easily).

i also noticed that a lot of the API documentation doesn't have the "stable since" information (cf #106) anymore, including this API.

What other resources are available?
as usual, ES has a nice page on it :)

[hdhalter]

Thanks for submitting this issue, @rursprung ! We are turning our focus to missing APIs soon, so we can address this then. In the meantime, if you want to take a stab at documenting it, please do : ).

I read through the discussion on 'stable since' and it sounded like the consensus was to not use that terminology and instead only document anything breaking?