v4

There is big list of changes and presumably breaking changes as well.

• Fixes in search object serialization and index creation.
• Fix in file-tree menu rendering. Breaking change. #70
• Site logo feature. #71
• New katex shortcode. KaTeX.
• New hint shortcode
• Table of content is now enabled for posts as well.
• Improved SCSS file layout allowing easier vars/styles overrides
• SCSS plugins: dark theme and numbered markdown headings.
• Search is migrated to flexsearch adding multi-language support.
• Also many small fixes and html and css.
• Updates to documentation