Create formatting guidance for docs

[timbussmann]

This issue is raised as an outcome of the discussion on this docs PR: https://github.com/Particular/docs.particular.net/pull/4489

The writing style guidance docs states the following for "language preference":

For consistency, prefer American English.

This guidance was interpreted to be also applicable for formatting style in the PR mentioned on the top of this issue. I think this shouldn't be the case as using a specific language doesn't imply using the American regional formatting conventions. Using American formatting can be confusing to our international audience as different countries can interpret data very differently, e.g. see this table: https://docs.oracle.com/cd/E19455-01/806-0169/overview-9/index.html

In order to make docs easy to read for an international audience, we should try to avoid any form of ambiguity on formatting styles when possible or when not possible a clear description of the used formatting style.

[timbussmann]

specifically I'd propose to use   instead of , for grouping of numbers