Add new CSS styles and JavaScript functionality for improved document presentation and navigation

- Created common_spec_style.css and common_spec_style_pdf.css for consistent styling across print and screen views.
- Introduced custom_spec_style.css for specific document formatting, including margins and header content.
- Added excerpt_explorer.css for notable sections with distinct styling.
- Implemented remove_links.css to disable link interactions and maintain visual consistency.
- Developed sidebar.css for a fixed sidebar navigation with responsive design adjustments.
- Added 3gpp.js for scroll spy functionality in the sidebar, enhancing user navigation experience.
- Introduced tex_eqn_to_svg.js for converting TeX equations to SVG format.
- Added multiple image assets to the media directory for use in documentation.
- Included logos for 3GPP and 5G in the media directory.