docs: rewrite Participate fee docs around Fee Juice, add Shield and Azguard onboarding

[AztecBot]

Improves the user-facing "Participate" docs ahead of the V5 public release, centered on the Fee Juice page served at https://docs.aztec.network/participate/basics/fees. The Participate section is non-versioned Docusaurus markdown built from docs/docs-participate/, so these source edits are what the site regenerates from — nothing gets replaced on the next build.

What changed

basics/fees.md (rewritten)

• Opens with a "What is Fee Juice?" section: a plain-language explanation ("prepaid balance for using the network") of how $AZTEC on Ethereum becomes Fee Juice on Aztec via the Fee Juice Portal, with the properties users must understand: 1:1 conversion (no exchange rate or slippage), non-transferable, one-way (no withdrawal), and publicly visible balance. Verified against l1-contracts/src/core/messagebridge/FeeJuicePortal.sol and noir-projects/noir-contracts/contracts/protocol/fee_juice_contract/src/main.nr (no transfer/withdraw paths; portal outflow only via distributeFees).
• Adds a mermaid flowchart of the $AZTEC → portal → claim (1:1) → Fee Juice → fees lifecycle, including the protocol-level payout of locked $AZTEC to sequencers/provers (same diagram conventions as token/economics.md).
• New "How to get Fee Juice" walkthrough: install Azguard (browser-extension Aztec wallet, connects to apps like MetaMask does on Ethereum), then bridge via Shield (bridge, with bridge+swap rolling out).
• Escapes \$AZTEC in the two paragraphs that mention it twice: the site enables remark-math + KaTeX, so a paragraph containing two bare $ tokens was being parsed as inline math and rendered as garbled italics on the live page.
• "Do you always need Fee Juice?" restructured from three equal-weight H3s into a weighted numbered list (pay directly as the default, app-pays via FPCs with sponsored/third-party as sub-bullets, pay privately) so the reading order matches importance.
• Testnet guidance now points at the Fee Juice faucet (aztec-faucet.nethermind.io), reconciling the previous discrepancy with docs-developers/getting_started_on_testnet.md, and keeps the "Fee Juice is not the AZTEC token" clarification.

Private fee payment reference moved to its new home
The community private FPC now lives in the private-fee-juice package (@alejoamiras/private-fee-juice on npm), so all DeFi Wonderland aztec-fee-payment references are replaced with the new package links (repo root, SDK README, product-requirements doc, PrivateFPC Noir source, private.test.ts — all sub-paths verified against the new repo) in:

• docs-participate/basics/fees.md
• docs-developers/docs/aztec-js/how_to_use_private_fee_juice.md
• developer_versioned_docs/version-v5.0.1/docs/aztec-js/how_to_use_private_fee_juice.md (so the live v5.0.1 dev docs update without waiting for a new version cut)

Wonderland references to other packages (aztec-immutables-macro, aztec-standards/AIP-20) are left untouched; those are separate tools.

Supporting pages (aligned terminology + first-ever Azguard/Shield references in the docs):

• basics/wallets.md: adds Azguard as the currently available wallet, ahead of the CLI wallet.
• basics/bridging.md: names the Fee Juice Portal, states its one-way/non-transferable nature, and points "Using Bridges" at Shield + Azguard.
• index.md, basics/index.md, token/index.md: fee blurbs now say fees are paid in Fee Juice (bridged from $AZTEC) and link to the fees page.
• docs-words.txt: adds azguardwallet and alejoamiras for cspell.

Verification

• yarn preprocess and full yarn build pass; built fees page contains no KaTeX math spans (the only remaining katex references are the site-wide stylesheet link), the new anchors resolve, and the mermaid block is emitted identically to the existing economics-page diagram.
• cspell clean on all touched files; zero wonderland occurrences remain in the fee-payment docs.