Static links to images should be relative to the REPO not pointing elsehere

[DuaneEllis-TI]

This occurs with the main readme.md file.

Specifically the large block diagram, the link points to:
(note: I have to mangle the LINK otherwise the ISSUE tracker treats this as a real link)
-> OTBR Architecture ->
HTTPS://gist.githubusercontent.com/Vyrastas/0489e2c081c0444c2462dcee54962cb7/raw/100a8a08f1e9103672017f0f05eac6f85cb8e31a/otbr-arch-borderagent_2x.png

Image links currently links point to an incomprehensible location sort of like: http://blahblah...googleusercontent...blabblah/somename/commitid/blahblah

They should instead be (a) part of the overall archive, and (b) be relative links not absolute links to files elsewhere (ie: Self contained)

[DuaneEllis-TI]

I think this can be closed - as all docs have moved to the openthread.io wiki