add how-to-open-urls-in-other-qubes

[ghost]

following up QubesOS/qubes-issues#8166 (comment)

@andrewdavidwong

• feel free to edit out stuff, it might be faster than back and forth comments.
• my English phrasing/grammar is sometimes er... weird, so don't worry to fix whatever you want.
• doc ref number: TBD
• before delving into the details let me know if some sections should move back to the original community doc (I tried to reflect your post)
• I updated the RPC policies to 4.x and tested them but it would be nice to have someone else confirm I haven't made any glaring mistake.
• once the doc is final I'll update the community doc as well.

[andrewdavidwong]

(Side note: Don't worry about the text wrapping. I independently decided today to try out an unwrapped text policy. You did the right thing.)

[andrewdavidwong]

• I updated the RPC policies to 4.x and tested them but it would be nice to have someone else confirm I haven't made any glaring mistake.

Perhaps @marmarek can have a look at it during review.