release-process: mention the code freeze, do not merge master after the first beta

[maelvls]

Preview	https://deploy-preview-585--cert-manager.netlify.app/docs/contributing/release-process/

[jetstack-bot]

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[maelvls]

@irbekrm Looks like the linter fails, and I can't change release-next from this PR 😞 Should I open another PR on the release-next branch?

next-docs/release-notes/release-notes-1.4/index.html
  hash does not exist --- next-docs/release-notes/release-notes-1.4/index.html --> ../../../docs/usage/certificate/#renewal
  target does not exist --- next-docs/release-notes/release-notes-1.4/index.html --> ../../../docs/installation/upgrading/remove-deprecated-apis/#upgrading-existing-cert-manager-resources

Update: I created #586 to unblock the situation

[maelvls]

/test pull-cert-manager-website-verify

[jetstack-bot]

@maelvls: The following test failed, say /retest to rerun all failed tests:

Test name	Commit	Details	Rerun command
pull-cert-manager-website-verify	ec34383	link	/test pull-cert-manager-website-verify

Full PR test history. Your PR dashboard. Please help us cut down on flakes by linking to an open issue when you hit one in your PR.

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[maelvls]

I need to update this with what to do for "subsequent betas".

[maelvls]

I created a branch directly on the repo. I recreated #607 from my branch instead.