Add Dark Mode feature documentation

[OUMIMANDAL]

Enhance Dark Mode Toggle

This PR builds on the existing dark mode toggle (#32) and adds:

Adds a dedicated DarkMode.md documentation page, which includes:

Feature overview

Usage instructions

Keyboard shortcut (Ctrl+D / Cmd+D) info

Persistence behavior via localStorage

Accessibility and color contrast notes

Minor UI adjustments and code clean-up in Navbar.tsx to ensure smooth integration.

Why this PR is needed:

The core Dark Mode toggle exists, but there was no official documentation or keyboard accessibility.

Provides clear guidance for users and future contributors.

Improves the overall usability and maintainability of the feature.

Testing Instructions:

Start the dev server: npm run dev or yarn dev

Open http://localhost:3000/

Click the Dark Mode toggle button → theme switches correctly

Use the keyboard shortcut Ctrl+D / Cmd+D → theme toggles

Refresh the page → selected theme persists

Verify button text, layout, and readability in both Light and Dark modes

This PR adds detailed documentation for the Dark Mode feature.
See the full guide here: https://github.com/vatsalsinghkv/portfolio-website/blob/darkmode-enhancement-test/docs/DarkMode.md

Notes:

Complements PR #32 by providing full documentation, accessibility notes, and UX enhancements.

No breaking changes introduced.
(reference: issue #31)

[vercel]

@OUMIMANDAL is attempting to deploy a commit to the vatsalsinghkv's projects Team on Vercel.

A member of the Team first needs to authorize it.

[OUMIMANDAL]

Hi @vatsalsinghkv, just to clarify: the failing Vercel check is only due to deployment authorization and not a code issue. The Dark Mode enhancements (keyboard shortcut, smooth transition, and documentation) are fully implemented and tested.

References: #31, #32