docs: more S2 docs fixes (#8944)
* add horizontal scrolling to MobileSearchMenu's TagGroup
* remove test index page
* Remove placeholder from S2 migration guide
* update markdown docs script to handle more custom components
* more updates to markdown docs with custom handling for style props docs
* add visibility to s2 docs
* add fixed width to s2 accordion example
* add Picker migration docs for defaultSelectedKey and selectedKey
* remove autoFocus from search menu SearchField to prevent focus ring flicker
* only render ToC header if there are sections
* auto-focus search menu field
* add index page
* add fuzzy searching and tags to search
* remove extra 'Esc' key handler (fixes view transition issue)
* show Introduction pages first in the menu card list
* remove fuzzy search
* improve mobile search menu experience
* fix relative link on index page
* fix mobile search menu scrolling
* mobile search: fix scroll to top when searching
* mobile search: improve tag group container
