Move experimental features to specs/_features folder

Author: hwwhww

specs/custody_game/beacon-chain.md …s/_features/custody_game/beacon-chain.mdspecs/custody_game/beacon-chain.md renamed to specs/_features/custody_game/beacon-chain.md specs/custody_game/beacon-chain.md renamed to specs/_features/custody_game/beacon-chain.md

@@ -36,11 +36,11 @@ docs are requisite for this document and used throughout. Please see the Custody
 
 ## Becoming a validator
 
-Becoming a validator in Custody Game is unchanged from Phase 0. See the [Phase 0 validator guide](../phase0/validator.md#becoming-a-validator) for details.
+Becoming a validator in Custody Game is unchanged from Phase 0. See the [Phase 0 validator guide](../../phase0/validator.md#becoming-a-validator) for details.
 
 ## Beacon chain validator assignments
 
-Beacon chain validator assignments to beacon committees and beacon block proposal are unchanged from Phase 0. See the [Phase 0 validator guide](../phase0/validator.md#validator-assignments) for details.
+Beacon chain validator assignments to beacon committees and beacon block proposal are unchanged from Phase 0. See the [Phase 0 validator guide](../../phase0/validator.md#validator-assignments) for details.
 
 ##### Custody slashings
 

specs/custody_game/validator.md specs/_features/custody_game/validator.mdspecs/custody_game/validator.md renamed to specs/_features/custody_game/validator.md

@@ -143,7 +143,7 @@ If the node does not already have connected peers on the topic it needs to sampl
 
 ### Topics and messages
 
-Following the same scheme as the [Phase0 gossip topics](../phase0/p2p-interface.md#topics-and-messages), names and payload types are:
+Following the same scheme as the [Phase0 gossip topics](../../phase0/p2p-interface.md#topics-and-messages), names and payload types are:
 | Name                             | Message Type              |
 |----------------------------------|---------------------------|
 | `das_sample_{subnet_index}`      | `DASSample`               |
@@ -192,7 +192,7 @@ This is to serve other peers that may have missed it.
 
 To pull samples from nodes, in case of network instability when samples are unavailable, a new query method is added to the Req-Resp domain.
 
-This builds on top of the protocol identification and encoding spec which was introduced in [the Phase0 network spec](../phase0/p2p-interface.md). 
+This builds on top of the protocol identification and encoding spec which was introduced in [the Phase0 network spec](../../phase0/p2p-interface.md). 
 
 Note that DAS networking uses a different protocol prefix: `/eth2/das/req`
 

specs/das/das-core.md specs/_features/das/das-core.mdspecs/das/das-core.md renamed to specs/_features/das/das-core.md

@@ -39,7 +39,7 @@ The adjustments and additions for Shards are outlined in this document.
 
 ### Topics and messages
 
-Following the same scheme as the [Phase0 gossip topics](../phase0/p2p-interface.md#topics-and-messages), names and payload types are:
+Following the same scheme as the [Phase0 gossip topics](../../phase0/p2p-interface.md#topics-and-messages), names and payload types are:
 
 | Name                            | Message Type             |
 |---------------------------------|--------------------------|

specs/das/fork-choice.md specs/_features/das/fork-choice.mdspecs/das/fork-choice.md renamed to specs/_features/das/fork-choice.md

@@ -33,7 +33,7 @@ This document represents the changes to be made in the code of an "honest valida
 
 ## Prerequisites
 
-This document is an extension of the [Bellatrix -- Honest Validator](../bellatrix/validator.md) guide.
+This document is an extension of the [Bellatrix -- Honest Validator](../../bellatrix/validator.md) guide.
 All behaviors and definitions defined in this document, and documents it extends, carry over unless explicitly noted or overridden.
 
 All terminology, constants, functions, and protocol mechanics defined in the updated Beacon Chain doc of [Sharding](./beacon-chain.md) are requisite for this document and used throughout.