docs: add Solana gasless wallet module

[ihsraham]

Summary

• Add docs for the Solana gasless wallet module: overview, setup, configuration, API reference, account management, balances, send/sign flows, SPL transfers, message verification, and errors.
• Wire the module into wallet-module navigation, module indexes, flat URL redirects, and redirect validation.
• Add a local source-watch script for tetherto/wdk-wallet-solana-gasless open PRs.

Source context

• Checked tetherto/wdk-wallet-solana-gasless#1, which is merged and introduced the initial module.
• Checked open module PRs #2 and #3; #2 adds transactionMaxFee, so these docs describe the merged behavior and note that send/sign do not enforce a transaction fee cap yet.
• Keep this PR in draft until package publish/version alignment is confirmed. Public npm currently reports 0.0.0; source package.json is 1.0.0-beta.1.
• The merged source uses Kora-compatible paymaster APIs, so the docs use that terminology.

Source watch

• scripts/check-solana-gasless-source-prs.mjs lists every open PR in tetherto/wdk-wallet-solana-gasless, so source PRs opened later are picked up whenever the local check is run.
• Keep this docs PR in draft until the source PRs are merged and the docs are refreshed.
• Latest local run found current blockers: tetherto/wdk-wallet-solana-gasless#2 and tetherto/wdk-wallet-solana-gasless#3.

Validation

• git diff --check
• node --check scripts/check-solana-gasless-source-prs.mjs
• npm run check:meta
• npm run check:redirects
• LINK_CHECK_EXTERNAL=false npm run check:links
• npm run build
• npm run check:solana-gasless-source (expected nonzero while source PRs are open; found #2 and #3)