Update Lighthouse book Sec 3-6 and FAQ (#4221)

## Issue Addressed

Update Lighthouse book to include latest information especially after Capella upgrade

## Proposed Changes

Notable changes:
- Combine Sec 4.1 & 6.1 into Sec 4, because Sec 6.1 is importing validator key which is a required step when want to run a validator
- Combine Sec 5.1 & 5.2 with Sec 5, and move Sec 5 to under Sec 9
- Added partial withdrawals in Sec 6



## Additional Info

Please provide any additional information. For example, future considerations
or information useful for reviewers.


Co-authored-by: chonghe <tanck2005@gmail.com>

book/src/SUMMARY.md

@@ -11,15 +11,10 @@
     * [Update Priorities](./installation-priorities.md)
 * [Run a Node](./run_a_node.md)
 * [Become a Validator](./mainnet-validator.md)
-    * [Become a Testnet Validator](./testnet-validator.md)
-* [Key Management](./key-management.md)
-    * [Create a wallet](./wallet-create.md)
-    * [Create a validator](./validator-create.md)
-    * [Key recovery](./key-recovery.md)
 * [Validator Management](./validator-management.md)
-    * [Importing from the Staking Launchpad](./validator-import-launchpad.md)
     * [Slashing Protection](./slashing-protection.md)
     * [Voluntary Exits](./voluntary-exit.md)
+    * [Partial Withdrawals](./partial-withdrawal.md)
     * [Validator Monitoring](./validator-monitoring.md)
     * [Doppelganger Protection](./validator-doppelganger.md)
     * [Suggested Fee Recipient](./suggested-fee-recipient.md)
@@ -46,6 +41,8 @@
     * [Remote Signing with Web3Signer](./validator-web3signer.md)
     * [Database Configuration](./advanced_database.md)
     * [Database Migrations](./database-migrations.md)
+    * [Key Management](./key-management.md)
+    * [Key Recovery](./key-recovery.md)
     * [Advanced Networking](./advanced_networking.md)
     * [Running a Slasher](./slasher.md)
     * [Redundancy](./redundancy.md)