fix: made documentation more comprehensive

[DeVikingMark]

PR-Codex overview

This PR focuses on improving clarity and consistency in documentation across various markdown files related to the @farcaster/hub-web and @farcaster/hub-nodejs packages.

Detailed summary

• Updated section title from "Getting start" to "Getting started" in packages/hub-web/README.grpcweb.md.
• Changed "Jan 1, 2021" to "January 1, 2021, at 00:00:00 UTC" in packages/hub-nodejs/docs/Utils.md.
• Revised phrasing for clarity regarding epoch usage in packages/hub-nodejs/docs/Utils.md.
• Removed unnecessary articles in descriptions of Ed25519Signer and Eip712Signer in packages/hub-nodejs/docs/Builders.md.

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