Add Run a Node guide (#3681)

## Issue Addressed

Related to #3672  

## Proposed Changes

- Added a guide to run a node. Mainly, copy and paste from 'Merge Migration' and 'Checkpoint Sync'.
- Ranked it high in ToC:
  - Introduction
  - Installation
  - Run a Node
  - Become a Validator
	...
- Hid 'Merge Migration' in ToC.

## Additional Info

- Should I add/rephrase/delete something?
- Now there is some redundancy:
  - 'Run a node' and 'Checkpoint Sync' contain similar information.
  - Same for 'Run a node' and 'Become a Validator'.


Co-authored-by: kevinbogner <114221396+kevinbogner@users.noreply.github.com>
Co-authored-by: Michael Sproul <micsproul@gmail.com>

book/src/SUMMARY.md

@@ -1,9 +1,6 @@
 # Summary
 
 * [Introduction](./intro.md)
-* [Become a Validator](./mainnet-validator.md)
-    * [Become a Testnet Validator](./testnet-validator.md)
-* [Merge Migration](./merge-migration.md)
 * [Installation](./installation.md)
     * [System Requirements](./system-requirements.md)
     * [Pre-Built Binaries](./installation-binaries.md)
@@ -13,6 +10,9 @@
     * [Cross-Compiling](./cross-compiling.md)
     * [Homebrew](./homebrew.md)
     * [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)
@@ -46,6 +46,7 @@
     * [Pre-Releases](./advanced-pre-releases.md)
     * [Release Candidates](./advanced-release-candidates.md)
     * [MEV and Lighthouse](./builders.md)
+    * [Merge Migration](./merge-migration.md)
 * [Contributing](./contributing.md)
     * [Development Environment](./setup.md)
 * [FAQs](./faq.md)