Fix image links in markdown files

[liamfallon]

This PR fixes the image links in the markdown files in the Nephio documentation. Prior to this PR, the images did not appear whent he markdown was rendered.

[liamfallon]

/assign @CsatariGergely @tliron @efiacor

[nephio-prow]

@liamfallon: GitHub didn't allow me to assign the following users: CsatariGergely.

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/assign @CsatariGergely @tliron @efiacor

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[liamfallon]

/assign @lapentad

[efiacor]

/approve
/lgtm

[nephio-prow]

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