docs: link the Sway docs hub from the Standard Library page (closes #4194)

[wrvnnull]

Summary

Resolves #4194 by adding a link to the full Sway documentation hub
(fuellabs.github.io/sway) from the Standard Library introduction page, so
forc-doc readers can reach the Sway Book, Forc reference, and library API docs
in one click.

Changes

• docs/book/src/introduction/standard_library.md - add docs hub link

Test plan

• Markdown link verified

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[cursor]

PR Summary

Low Risk
Documentation-only markdown change with no runtime or build impact.

Overview
Adds a short paragraph on the Standard Library introduction page that points readers to the central Sway documentation site at fuellabs.github.io/sway, covering the Sway Book, Forc reference, and library API docs alongside the existing std repo and API doc links.

Addresses #4194 so forc-doc / book readers can jump to the full docs hub in one click.

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[wrvnnull]

Re-triggering CLA check after signing the Fuel Labs Contributor License Agreement. Thanks!

[wrvnnull]

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