site: Add docs on how to rotate gRPC certs

[stevesloka]

Fixes #1857 by documenting how to rotate certs between Envoy<>Contour.

Signed-off-by: Steve Sloka [email protected]

[codecov]

Codecov Report

Merging #2282 into master will increase coverage by 0.09%.
The diff coverage is 68.46%.

@@            Coverage Diff             @@
##           master    #2282      +/-   ##
==========================================
+ Coverage   77.35%   77.45%   +0.09%     
==========================================
  Files          58       59       +1     
  Lines        5154     5234      +80     
==========================================
+ Hits         3987     4054      +67     
- Misses       1080     1092      +12     
- Partials       87       88       +1

Impacted Files	Coverage Δ
internal/k8s/syncer.go	0% <0%> (ø)
cmd/contour/serve.go	0% <0%> (ø)	⬆️
cmd/contour/contour.go	3.84% <0%> (-0.11%)	⬇️
internal/dag/cache.go	88.59% <100%> (+2.32%)	⬆️
internal/contour/handler.go	74.52% <100%> (-0.93%)	⬇️
cmd/contour/servecontext.go	94.54% <100%> (+0.05%)	⬆️
internal/k8s/converter.go	55.95% <90%> (+8.98%)	⬆️
internal/k8s/kind.go	100% <0%> (+10%)	⬆️

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[youngnick]

LGTM