docs: add Solana as payment method

[lgalabru]

Summary

Add Solana to the docs navigation and payment-method docs surface.

Changes

• add Solana to the sidebar under Payment Methods, after Lightning
• add Solana payment method overview page
• add Solana charge page
• wire Solana into payment-method cards, charge intent cards, and OG subcategory mapping
• link Solana session to the in-progress spec PR instead of publishing a local session docs page
• clarify Solana overview wording around split payouts, fee payer support, and coming-soon session support

Notes

• Solana session is intentionally not documented as a local page yet
• the Solana session card currently points to the spec draft PR:
  • Solana session intent specification mpp-specs#201

Validation

• pnpm check:ci
• pnpm check:types
• pnpm build
• local docs server tested successfully

[vercel]

@lgalabru is attempting to deploy a commit to the Tempo Team on Vercel.

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[brendanjryan]

LGTM -- I will clean up the nit in another pr

[brendanjryan]

👍

[brendanjryan]

nit: can we add the bun group here as well?

e.g. https://mpp.dev/sdk/typescript#install