feat: Init Landingpage

[devdorn]

This pull request adds a new landing page for the JiraTime browser extension, including a modern HTML structure and custom CSS for a visually appealing and responsive design. The changes introduce a clear overview of the extension's features, installation instructions, and branding elements.

Landing page creation and design:

• Added a new docs/index.html file with a full landing page layout, including navigation, hero section, features list, installation steps, and footer.
• Introduced a custom stylesheet in docs/style.css to provide consistent theming, typography, layout, and responsive design for the landing page.

Content and user experience:

• Highlighted key features of JiraTime (Smart List, Live Timer, Quick Entry, Pinned Tasks) in a feature grid for developer-focused marketing.
• Detailed step-by-step installation instructions for users to easily set up the browser extension.
• Included branding elements, open source notice, and external links for project transparency and navigation.