docs: add Gateway contracts, link to corresponding doc pages

[derrekcoleman]

• Add BOB Gateway contracts to Contracts page
• Hyperlink relay and gateway words to corresponding docs (mainly so people don't think tBTC relay is tBTC token)
• Rearrange sections to put gateway and relay on top, then BOB, then ethereum (same for testnet section). This makes the diff look crazier than it is
• Add missing code format `` around address to the only section that didn't have it

Please take a quick peek at the Vercel preview to sanity check links, formatting 🙏

Closes #363

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Latest commit: 706e259

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Name	Status	Preview	Comments	Updated (UTC)
bob	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Oct 4, 2024 8:54pm