Add next release schedule

[sloosvel]

Description

This PR adds the new release schedule to the documentation

• Closes #3793

• Link to documentation: https://esmvaltool--3794.org.readthedocs.build/en/3794/community/release_strategy/release_strategy.html#upcoming-releases

• • *

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Checklist

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• [x] 🛠 This pull request has a descriptive title ~- [ ] 🛠 Code is written according to the code quality guidelines~
• [x] 🛠 Documentation is available
• [x] 🛠 Tests run successfully
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~### New or updated recipe/diagnostic~

~- [ ] 🧪 Recipe runs successfully~ ~- [ ] 🧪 Recipe is well documented~ ~- [ ] 🧪 Figure(s) and data look as expected from literature~ ~- [ ] 🛠 Provenance information has been added~

~### New or updated data reformatting script~

~- [ ] 🛠 Documentation is available~ ~- [ ] 🛠 The dataset has been added to the CMOR check recipe~ ~- [ ] 🛠 The dataset has been added to the shared data pools of DKRZ and Jasmin by the @ESMValGroup/OBS-maintainers team~ ~- [ ] 🧪 Numbers and units of the data look physically meaningful~

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[valeriupredoi]

@bouweandela wanna have a looksee and merge, pls, buds :beer: