Docs streamline fixes

[RobMulla]

Description

Title

Docs: Streamline "First Run" tutorial and "Tutorials" landing page

Description

Improves the "Day 0" experience by making the tutorials landing page more useful and the "First Run" tutorial copy-paste friendly.

Changes

• Landing Page (tutorials.md): Added a "New to MaxText?" section and descriptions for tutorial categories.
• First Run Tutorial (first_run.md):
  • Added export BUCKET_NAME to simplify copy-pasting.
  • Replaced links with explicit gcloud commands for bucket creation.
  • Added inline configs/base.yml snippet to show default model params (1B, float32).
  • Clarified execution paths (cd MaxText) and checkpoint usage for decoding.

Motivation

Smooths out friction points for new users - preventing common errors like FileNotFoundError (missing cd) or correct bucket permissions.

Verification

• Built and verified docs locally (sphinx-build).

Checklist

Before submitting this PR, please make sure (put X in square brackets):

• I have performed a self-review of my code. For an optional AI review, add the gemini-review label.
• I have necessary comments in my code, particularly in hard-to-understand areas.
• I have run end-to-end tests tests and provided workload links above if applicable.
• I have made or will make corresponding changes to the doc if needed, including adding new documentation pages to the relevant Table of Contents (toctree directive) as explained in our documentation.

[SurbhiJainUSC]

These are the outdated instructions to install MaxText. Can you update this and refer to https://maxtext.readthedocs.io/en/latest/install_maxtext.html#from-source?

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