[Edit] Stakeholder: Add content for richness

[AnamolR]

Description

This PR updates the existing Stakeholder entry in the uiux/concepts/ section to include two new content sections: Types of Stakeholders in UX Projects and Stakeholder Influence on UX.

Markdown file: stakeholder.md
The additions expand on stakeholder roles, categories, and how they shape product direction, design priorities, and constraints. This richer content improves clarity for learners working in cross-functional teams or collaborative UX environments.

Type of Change

Editing an existing entry (fixing a typo, bug, issues, etc)

Checklist

• All writings are my own.
• My edits follow the Codecademy Docs style guide.
• My changes generate no new warnings.
• I have performed a self-review of my own writing and code.
• I have checked the updated content and corrected any misspellings.
• I have made corresponding changes to the documentation if needed.
• I have confirmed my changes are not being pushed from my forked main branch.
• I have confirmed that I'm pushing from a new branch named after the changes I'm making.
• I have linked any issues that are relevant to this PR in the Issues Solved section.

Issues Solved

closes #6686