docs: Add an EVM/Solidity developer onboarding page (mental model mapping)

## Motivation

Developers coming from the EVM/Solidity world face a steep mental-model jump when approaching Miden: account-based global state vs. account-centric local state, global on-chain execution vs. client-side proving, `transfer()` calls vs. note creation/consumption, etc.

The get-started section explains Miden's concepts (accounts, notes, storage) well on their own terms, but there is no single page that maps familiar EVM/Solidity concepts to their Miden counterparts. Such a mapping page would significantly flatten the onboarding curve for the largest existing developer audience in web3.

## Proposed change

Add a new page under `docs/builder/get-started/` (working title: **"Coming from Ethereum / Solidity"**, e.g. `evm-developers.md`), containing:

1. **Concept mapping table** — a quick-reference cheat sheet, roughly:

| Ethereum / Solidity | Miden |
|---|---|
| Smart contract (code + storage) | Account (code + storage) |
| Storage slots / mappings | Account storage slots & storage maps |
| ERC-20 token | Fungible asset issued by a faucet account |
| `transfer(to, amount)` | Creating a P2ID note, which the recipient consumes |
| `msg.sender` | Account authentication component (e.g. signature scheme) |
| Events / logs | Notes (the primary communication primitive) |
| Global, on-chain execution | Local execution with a ZK proof submitted on-chain |
| Public mempool | Public vs. private notes; note transport for private data |

2. **Mental model shifts** — short paragraphs on the big differences:
- Note-based (UTXO-like) asset transfers vs. account-based balance updates
- Client-side proving & local state vs. global state machine replication
- Public and private state/notes

3. **Side-by-side code comparison** — a minimal Solidity ERC-20 `transfer()` next to its TypeScript web-client equivalent (creating/consuming a P2ID note), so readers can anchor new concepts to code they already understand.

4. **Where to go next** — relative links to the existing `accounts`, `notes`, `read-storage`, and smart-contract sections.

## Notes

- The get-started sidebar is autogenerated, so the page only needs frontmatter (`sidebar_position`) — no `sidebars.ts` change required.
- I will follow the repo conventions: run `npm run build` before opening the PR, use relative internal links, and keep the diff limited to the new page.

I'm happy to write this page and open the PR. Could a maintainer confirm the placement/title and assign this issue to me?