Add note for required web app manifest members

[dipikabh]

Summary

This PR adds a note to the manifest members that are required for a web app to be installable.

Bonus edit: In the icons file, I've rephrased the safari note so that we don't use "key" because we refer them as (manifest) members in content.

Supporting details

The following browser-specific information is good to be included in the BCD table (instead of being added as a note on respective manifest member pages):

Required manifest members:
Chromium-based browsers, including Google Chrome, Samsung Internet, and Microsoft Edge, require that the manifest includes the following members:

• name or short_name
• icons must contain a 192px and a 512px icon
• start_url
• display and/or display_override
• prefer_related_applications must be false or not present

Context: mdn/content#35847 (comment)

Assumptions and questions

• I am assuming that by virtue of "mirror" value, the added note will be replicated to all chromium-based browsers. Is that correct?
  I've added the note only to Chrome, but it also applies to Chrome Android, Edge, Opera (and Android), Samsung Internet, WebView Android.

• I am unclear about some of the values showing in the table. For example:

	chrome	edge	opera
display on MDN	39	79	26
display in BCD	39	mirror	mirror

I am not sure what's the source of the values visible on MDN.

[caugner]

• I am assuming that by virtue of "mirror" value, the added note will be replicated to all chromium-based browsers. Is that correct?

Yes, that's correct, mirror replicates the notes.