[ADD] reference/external_json_api

[Julien00859]

#14091

[robodoo]

[Julien00859]

Hello there @odoo/doc-review :)

We had to close 14091 because it was linked to a community branch and we wanted to merge the (r+ez) community branch without the documentation one. Turn out mergebot/runbot wants we open a new PR instead or re-opening 14091, loosing the preview reviews in the process. Sorry for that :(

• external_api.rst -> external_rpc_api.rst, with only a few changes, please only review what changed.
• external_api.rst, the new "json2" document, to be reviewed.

The new documentation will be part of my talk at OXP (Friday) so let's try and have it merged by then :D

[Julien00859]

retarget 19.0