feat(upgrade): embed upgrade.md in docs

[lahabana]

Fix #1535

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[slonka]

LGTM, but do we now need to use template variables in https://github.com/kumahq/kuma/blob/master/UPGRADE.md so that it renders correctly on both websites?

[slonka]

Rendered: https://deploy-preview-1706--kuma.netlify.app/docs/2.6.x/production/upgrades-tuning/upgrade-notes/

also some links are broken, should we disable link checker on this page?

[lahabana]

also some links are broken, should we disable link checker on this page?

I think we'd be better off fixing the links.

For the variable the UPGRADE pages are different between Kuma and KM and the one from KM will link to the Kuma docs (see: https://github.com/Kong/kong-mesh/blob/master/UPGRADE.md). It was an easy low effort change.
In the future we may want to make these pages better but at least now we have this info on the docs website (perfect is the enemy of good :) )

[lahabana]

This should fix the links: kumahq/kuma#9835

[slonka]

CoreDNS website timed out on the last link check and there is a problem with GH anchor that was fixed in main. Let's see if merge fixes it.

[slonka]

For the links issue: https://github.com/kumahq/kuma-website/pull/1712/files