chore: upgrade mdbook version

[Kelketek]

Description

This merge request upgrades mdbook and mdbook-mermaid.

Motivation and Context

The versions used are quite out of date, making the documentation for them quite out of date, and the available plugins much fewer.

How Has This Been Tested?

Locally, manually. Follow the directions in the README.md to give it a run.

[gemini-code-assist]

Code Review

This pull request cleans up the repository by removing Node.js build scripts, updating mdbook and mdbook-mermaid dependency versions, and escaping HTML angle brackets across several RFC markdown files to prevent rendering issues. It also refactors the Handlebars template (index.hbs) to use a new resource helper and inline SVG icons instead of FontAwesome. Feedback on the changes highlights syntax errors in index.hbs where variables are incorrectly passed as nested string templates to the resource helper. Additionally, a minor grammatical error was found in RFC-0331_Indexers.md, and an unescaped placeholder tag in RFC_template.md needs to be updated to prevent rendering issues.

Important

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[gemini-code-assist]

The Handlebars helper resource should take the variable favicon directly as an argument rather than as a nested string template. Using "{{ favicon }}" passes the literal string "{{ favicon }}" to the helper, which will fail to resolve the correct path.

<link rel="shortcut icon" href="{{ resource favicon }}">

[gemini-code-assist]

Similar to the favicon helper call, the this context variable should be passed directly to the resource helper without quotes or nested curly braces. Using "{{this}}" passes the literal string "{{this}}" instead of the actual item value in the loop.

<script type="text/javascript" src="{{ resource this }}"></script>

[gemini-code-assist]

There is a minor grammatical issue here: "The indexer library work" should be "The indexer library works" to match the singular subject "library".

Suggested change

	The indexer library work via three interoperating modules: substate scanning, substate decoding, and the substate
	The indexer library works via three interoperating modules: substate scanning, substate decoding, and the substate

[gemini-code-assist]

In this line, <COPYRIGHT HOLDER | The Tari Development Community> is not escaped or converted to parentheses, which will cause it to be parsed as an invalid HTML tag by mdbook and hidden in the rendered output. Since other placeholders in this file have been updated to use parentheses (e.g., (RFC-Number), (Short Title), (Maintainer name)), we should do the same here for consistency and to avoid rendering issues.

Suggested change

	Copyright &lt;YEAR&gt; <COPYRIGHT HOLDER | The Tari Development Community>
	Copyright (YEAR) (COPYRIGHT HOLDER | The Tari Development Community)