docs: clean up broken callout, dead links, and outdated wording

[memosr]

Summary

Four small but real documentation fixes across the repo.

Changes

1. README.md — Remove broken [!NOTE] callout block

The callout marker > [!NOTE] was followed by lines that did not start with the > prefix, breaking the GitHub callout syntax:

- > [!NOTE]
- To run the node using a supported client, you can use the following command:
- `CLIENT=supported_client docker compose up --build`
-
- Supported clients:
-  - reth (runs vanilla node by default...)
-  - geth
-  - nethermind

The same information is already present in the Quick Start (lines 34-38) and Supported Clients (lines 41-45) sections immediately above, so the redundant block was removed entirely.

2. reth/README.md — Replace "master README" with "root README"

- See hardware requirements mentioned in the master README
+ See hardware requirements in the root README

- The node follows the standard docker compose workflow in the master README.
+ The node follows the standard docker compose workflow in the root README.

"master" as a hierarchy term is ambiguous — readers might wonder if it refers to a branch name. "root README" is unambiguous and matches modern open source conventions.

3. CONTRIBUTING.md — Update deprecated Coinbase support URL

- https://support.coinbase.com/customer/en/portal/articles/2288496-how-can-i-contact-coinbase-support-
+ https://help.coinbase.com/

The old Zendesk-style /customer/en/portal/articles/ URL format has been retired. The modern entry point is help.coinbase.com.

4. SECURITY.md — Update dead OP Stack docs link

- [5]: https://docs.optimism.io/docs/releases/bedrock/
+ [5]: https://docs.optimism.io/stack/getting-started

The /docs/releases/bedrock/ path no longer exists in the current OP Stack documentation. Updated to the current canonical path.

Verification

• ✅ All four changes verified locally
• ✅ Replacement URLs all confirmed to load correctly
• ✅ README content preserved (duplicate block removed, not unique info)

[cb-heimdall]

🟡 Heimdall Review Status

Requirement	Status	More Info
Reviews	🟡 0/1	Denominator calculation Show calculation 1 if user is bot 0 1 if user is external 0 2 if repo is sensitive 0 From .codeflow.yml 1 Additional review requirements Show calculation Max 0 0 From CODEOWNERS 0 Global minimum 0 Max 1 1 1 if commit is unverified 1 Sum 2