Freeze the cert-manager 1.15 docs

[wallrj]

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Following the updated instructions from the release process:

(final release only) Freeze the docs/ folder by creating a copy , removing the
pages from that copy that don't make sense to be versioned, and updating the
manifest.json file:

export RELEASE=1.15
cp -r content/docs content/v${RELEASE}-docs
rm -rf content/v${RELEASE}-docs/{installation/supported-releases,installation/upgrading,release-notes,contributing}
sed -i.bak "s|docs/|v${RELEASE}-docs/|g" content/v${RELEASE}-docs/manifest.json
jq < content/v${RELEASE}-docs/manifest.json >/tmp/manifest \
  'del(.routes[0].routes[] | select(.title | test("Releases|Contributing")))'
mv /tmp/manifest content/v${RELEASE}-docs/manifest.json

I updated the instructions in the release process because it wasn't dropping the contributing section,
and it was mangling the api-docs.

[cert-manager-prow]

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