templates: add placeholder templates (footer.html, head.html)
templates: allow 'base_html', 'footer_html', 'head_html', 'header_html' to be added via wb-manager 'add-template' cmd
ui: fix logo path, support linking
ui: make url on banner an input field
docs: clarify docs around templates, paths
- add ui-overview as UI toc page
- refactor ui-customization to top-level customizations page
- add template-guide for in-depth template reference
- add new-vue-ui page for docs on new ui, with images
- fix adding logo to old ui, add to docs