fix: remove mdBook-style .html extensions from specs overview diagram links

[haileymarshall]

What changed? Why?

The Mermaid architecture diagrams on the protocol specs overview page contain 15 links that still use mdBook-style .html paths, left over from the specs migration (#1335). These paths don't resolve on the docs site, so every one of these links currently returns a 404:

Link in diagrams	Status
./batcher.html	404
./bridging/withdrawals.html#the-optimism-portal-contract	404
../reference/glossary.html#batcher-transaction	404
./proofs/contracts.html#disputegamefactory	404
./proofs/contracts.html#aggregateverifier	404

This PR drops the .html extension from each href so they match the working links already used in the same diagrams (e.g. ./consensus/, ./execution/).

Notes to reviewers

• Only href values inside the Mermaid diagrams changed; no diagram structure or prose was touched.
• All five destination pages exist, and the anchor headings (The Optimism Portal Contract, Batcher Transaction, DisputeGameFactory, AggregateVerifier) are present in the target pages.

How has it been tested?

• Confirmed each old .html URL returns 404 on docs.base.org and each corrected URL returns 200.
• Searched the repo for any other internal .html links; this file was the only one remaining.

[cb-heimdall]

🟡 Heimdall Review Status

Requirement	Status	More Info
Reviews	🟡 0/2	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