Access modifiers update

[ruturajjadhav07]

Description

Added a new entry for Java Access Modifiers under the content/java/concepts/access-modifiers/ directory.
This entry explains the four access modifiers (public, private, protected, default) in a beginner-friendly way with a comparison table.
The guide also includes concise explanations suitable for learners to understand visibility, encapsulation, and safe code design.

Issue Solved

Type of Change

• Adding a new entry
• Editing an existing entry (fixing a typo, bug, issues, etc)
• Updating the documentation

Checklist

• All writings are my own.
• My entry follows 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 my entry 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.