Add Intermediate PostgreSQL Questions and Update Sidebar

[mathncode-sid]

Description

This PR addresses Issue #33 by introducing a new set of 30 intermediate-level PostgreSQL questions and answers. Each question includes:

• A realistic scenario providing context.
• A clear question statement.
• The SQL query solution.
• A detailed explanation of the query.

Additionally, the _sidebar.md has been updated to include a link to this new section, ensuring learners can easily navigate to the intermediate questions.

Changes Made

1. Added intermediate-exercises.md:
   Contains 30 fully detailed PostgreSQL questions and answers in Markdown format.

2. Updated _sidebar.md:
   Added a link to the Intermediate Questions section under the PostgreSQL learning content for easy navigation.

Motivation

The beginner sections already provide fundamental PostgreSQL knowledge. These intermediate questions:

• Provide realistic scenarios for learners to understand practical database problems.
• Include SQL solutions with explanations, helping learners reason through queries.
• Expand the learning path from beginner to intermediate level.

Checklist

• Added intermediate_questions.md with 30 questions.
• Updated _sidebar.md to include a table of contents link.
• Verified Markdown formatting renders correctly.
• SQL queries tested for correctness on PostgreSQL 15.

Preview

• The Intermediate Questions section is now visible in the sidebar and fully accessible via GitHub Pages.
• Learners can navigate through questions with context, problem, SQL solution, and explanation.

Related Issue

• Resolves Issue #33

[fahimahammed]

please add video or screenshot of your changes

[fahimahammed]

Thank you for your contribution

[mathncode-sid]

please add video or screenshot of your changes

Let me do that right away.