docs: mention Int8 to represent i64

[saihaj]

Since @graphprotocol/graph-cli@0.51.0 we can use Int8 GraphQL Scalar type which is optimized for storage and is beneficial for most contracts that are indexing a i64 in Solidity.

Closes #396

[github-actions]

📦 Next.js Bundle Analysis for @graphprotocol/docs

This analysis was generated by the Next.js Bundle Analysis action. 🤖

One Page Changed Size

The following page changed size from the code in this PR compared to its base branch:

Page	Size (compressed)	First Load	% of Budget (350 KB)
/en/developing/creating-a-subgraph	26.69 KB	711.84 KB	203.38% (🟡 +0.03%)

Details

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