DOC Create unified release policy

[emteknetnz]

Issue https://github.com/silverstripeltd/product-issues/issues/908

Using in favour of #753 as this targets 5.4, then other one targeted 6.

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Copied from #753 (comment):

Things for Jenn to be aware of when reviewing and approving:

1. This is intended to blend the major release policy and minor release policy together.
2. It's not intended to actually change anything about our commitments, or about how we communicate those commitments, except that:
   1. The minor release policy says "Once a release candidate is tagged, only critical impact bug fixes can be added to the release" - this has been changed here to remove the word "critical" to better match reality.
   2. Wording has been updated to better clarify exactly what our commitments are. The interpretation of those commitments should not have changed, though.

[GuySartorelli]

LGTM assuming CI passes.
On to @SimulatedPanda to give approval.

[GuySartorelli]

Found one tiny thing when viewing the rendered copy

[emteknetnz]

I just changed " - Extended: Approximately one year, extending until the subsequent major release (e.g. CMS 5.4 will remain in partial support until CMS 7.0 is released)" to include "following subsequent" as it's a "double subsequent" jump

[GuySartorelli]

Changes look good. Just waiting for Jenn's review.

[GuySartorelli]

Minor change that I don't feel strongly about, but reading through again this stood out as potentially confusing.
These come from me, not Jenn

[GuySartorelli]

These are Jenn's review changes from slack.

[GuySartorelli]

Just #788 (comment) left to do

[GuySartorelli]

LGTM, just needs approval from Jenn

[SimulatedPanda]

I approve these changes and this combined policy can now be made live.