docs: Clarify how to find StarkGate bridge and token addresses in block explorers

[stoobie]

Description of the Changes

Readers repeatedly ask for the addresses for StarkGate bridge and supported tokens. This PR clarifies how to find the json files that include that information.

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Bridged tokens
Supported tokens and token addresses in StarkGate

Check List

• Changes have been done against main branch, and PR does not conflict
• PR title follows the convention: <docs/feat/fix/chore>(optional scope): <description>, e.g: fix: minor typos in code

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This change is 

[github-actions]

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[starknet-bot]

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[natan-granit]

We should have another section in the overview that is specific for StarkGate addresses.
It should have the same information as in the table currently in the StarkGate tokens sections. It should be the first section.

[starknet-bot]

Your preview build is ready! ✨ Check the following link in 1-2 minutes: https://starknet-io.github.io/starknet-docs/pr-1241/documentation/ .

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[starknet-bot]

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[natan-granit]

LGTM

[starknet-bot]

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