feat: Add Mempool Specification to Zebra Book

[crStiv]

Motivation

• Document the Zebra mempool architecture, behavior, and interfaces
• Create a comprehensive specification for developers
• Close issue Specify the Zebra mempool #9302: "Specify the Zebra mempool"

Solution

• Added detailed mempool specification document in the Developer Documentation section of the Zebra book
• Created a mermaid diagram illustrating the mempool architecture and transaction flow
• Updated the book's SUMMARY.md to include links to new documentation

Tests

• Verified that markdown formatting is correct
• Checked that internal links are valid
• Ensured the mermaid diagram syntax is correct and will render properly

Specifications & References

• Based on current Zebra mempool implementation
• Referenced ZIP-401 where appropriate

PR Checklist

• The PR name is suitable for the release notes.
• The solution is tested.
• The documentation is up to date.
• The PR has a priority label.
• If the PR shouldn't be in the release notes, it has the C-exclude-from-changelog label.

[oxarbitrage]

Hi, thank you for making this, i think is very useful.

In a high level overview i found a few things that we might want to fix.

[oxarbitrage]

I am having some issues visualizing the flowchart:

• Unsupported markdown in some of the descriptions: Remove lists from them.
• Colors in a black background: You can use different node shapes instead of colors: https://mermaid.js.org/syntax/flowchart.html#node-shapes

[crStiv]

Hi, thank you for making this, i think is very useful.

In a high level overview i found a few things that we might want to fix.

@oxarbitrage made changes as you suggested, what are ur thoughts?