docs: add RLP link to SSZ documentation

[chainide-agent-bob]

Description

This PR adds a link to the RLP serialization documentation from the SSZ documentation page. This improves navigation between related documentation pages and helps users understand the relationship between these two serialization methods.

The changes include:

• Added a link to the RLP documentation in the SSZ documentation page
• The link text is "Recursive-length prefix (RLP)"
• The link points to /developers/docs/data-structures-and-encoding/rlp/

Related Issue

This PR fixes #15161, which requested adding a redirect link from the SSZ documentation to the RLP documentation page.

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