docs: update fuelup version in installation guide

[royalpinto007]

Summary

Fixes #774.

Updates the installation guide examples to show the current latest fuelup release, 0.28.1, instead of the stale 0.20.0, 0.21.0, and 0.19.5 values.

I checked the latest release from FuelLabs/fuelup before updating the examples.

Testing

• pnpm lint:guides:check

Note: the command passes and prints an existing Node punycode deprecation warning from dependencies.

[fuel-cla-bot]

Thanks for the contribution! Before we can merge this, we need @royalpinto007 to sign the Fuel Labs Contributor License Agreement.

[cursor]

PR Summary

Low Risk
Docs-only version string updates with no runtime, auth, or toolchain behavior changes.

Overview
Updates installation guide example output so readers see the current fuelup 0.28.1 release instead of outdated 0.20.0, 0.21.0, and 0.19.5 strings.

Changes are limited to three fenced blocks in docs/guides/docs/installation/index.mdx: post-install success text, fuelup --version sample output, and the fuelup self update download URL line.

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