Add sidebar navigation to student help page and mini table of contents to features page

[Alexa-Solorzano]

These improvements make long help pages easier to navigate and improve usability for students and instructors.

• Added a sidebar "Browse by topic" navigation to the Student Help Page
• Added a mini table of contents to the Features Page to allow quick jumps to each feature
• Updated corresponding HTML and SCSS files for layout, spacing, and responsive behavior
• Ensured IDs are in place for smooth anchor scrolling
• Added scroll() helper method for smooth navigation

[github-actions]

Hi @Alexa-Solorzano, thank you for your interest in contributing to TEAMMATES!
However, your PR does not appear to follow our contribution guidelines:

• Title must start with the issue number the PR is fixing in square brackets, e.g. [#<issue-number>]
• Description must reference the issue number the PR is fixing, e.g. Fixes #<issue-number> (or Part of #<issue-number> if the PR does not address the issue fully)

Please address the above before we proceed to review your PR.