doc: add missing ADR files (#976)

docs/architecture/0020-rework-dao-voting.md

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+# 20. Harden closing of a DAO motion voting
+
+Date: 2024-05-02
+
+## Status
+
+Accepted
+
+## Context
+
+See [here](https://github.com/threefoldtech/tfchain/issues/889) for more
+details.
+
+## Decision
+
+In `propose()` extrinsic, set a minimum threshold `MotionMinThreshold` of 5
+votes for a motion to be proposed and if threshold is lower then return error
+`TresholdTooLow`. Also in `propose()`extrinsic, add a minimum motion duration of
+1 day and return error `InvalidProposalDuration` if optional duration is not set
+in the 1 day - 30 days interval.

docs/architecture/0021-allow-hdd-only-nodes.md

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+# 21. Allow 'only hdd' nodes to register on chain
+
+Date: 2024-05-22
+
+## Status
+
+Accepted
+
+## Context
+
+See [here](https://github.com/threefoldtech/tfchain/issues/967) for more
+details.
+
+## Decision
+
+Similar to what already exists for SSD, add a minimum HDD size requirement (also
+100 GB) on `resources.validate_hru()`. Then, on validating, make sure node has
+at least 1 minimum storage capacity available.