document how to include images into a form (online / offline)

[vlukashov]

As a developer I want the form documentation to cover file uploads, so that I know how to include an image into my form (e.g. an avatar image into the user profile form, or a photo into a workspace inspection form).

DoD:

• the "Creating Client-side forms" docs chapter includes an example of how to include an image into a form
• the (not-yet-existing) documentation of offline forms submission includes an example of how to submit a form with an image when offline
• the vaadin-upload documentation page links to the online and offline examples

Implementation notes:
The working prototype of offline form submission handles images as base64-encoded strings. This approach does not require changes to the endpoints API or the TS form binder API, but it clearly has limitations (e.g. it's not possible to stream uploads). In scope of this tasks it's OK to use the same approach as long as the limitations are explicitly listed in the documentation.

Working prototype:
vaadin-learning-center/crm-tutorial-typescript@29086d5