Update beginner quickstart guide

[Ei-Sandi]

Description

This PR updates contributing-docs/03a_contributors_quick_start_beginners.rst to fix outdated instruction and add missing prerequisites for running local tests.

1. Update Default Terminal Multiplexer: Clarified that Breeze now uses mprocs as the default multiplexer instead of tmux, and added instructions on how to use tmux via the --terminal-multiplexer tmux option if preferred.
2. Fix Breeze Exit Instructions: Updated the guide from the outdated stop_airflow command to reflect that users need to press q to quit, and added instructions to run breeze down afterward to properly spin down containers.
3. Add Missing Pre-commit Prerequisites: Added a relative link to the virtual environment setup guide right before the prek --all-files step. Previously, the guide lacked instructions to configure the .venv and install dependencies first, which caused pre-commit validation tests to fail for new users.

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Was generative AI tooling used to co-author this PR?

• Yes (please specify the tool below)
  (Gemini 3.5 flash - to draft this PR description.)

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• Read the Pull Request Guidelines for more information. Note: commit author/co-author name and email in commits become permanently public when merged.
• For fundamental code changes, an Airflow Improvement Proposal (AIP) is needed.
• When adding dependency, check compliance with the ASF 3rd Party License Policy.
• For significant user-facing changes create newsfragment: {pr_number}.significant.rst, in airflow-core/newsfragments. You can add this file in a follow-up commit after the PR is created so you know the PR number.

[boring-cyborg]

Congratulations on your first Pull Request and welcome to the Apache Airflow community! If you have any issues or are unsure about any anything please check our Contributors' Guide
Here are some useful points:

• Pay attention to the quality of your code (ruff, mypy and type annotations). Our prek-hooks will help you with that.
• In case of a new feature add useful documentation (in docstrings or in docs/ directory). Adding a new operator? Check this short guide Consider adding an example Dag that shows how users should use it.
• Consider using Breeze environment for testing locally, it's a heavy docker but it ships with a working Airflow and a lot of integrations.
• Be patient and persistent. It might take some time to get a review or get the final approval from Committers.
• Please follow ASF Code of Conduct for all communication including (but not limited to) comments on Pull Requests, Mailing list and Slack.
• Be sure to read the Airflow Coding style.
• Always keep your Pull Requests rebased, otherwise your build might fail due to changes not related to your commits.
  Apache Airflow is a community-driven project and together we are making it better 🚀.
  In case of doubts contact the developers at:
  Mailing List: dev@airflow.apache.org
  Slack: https://s.apache.org/airflow-slack

[Prab-27]

Cool ! Overall LGTM !