Add AsciiDoc files

[colleenmcginnis]

Preview link: https://search-ui_bk_1097.docs-preview.app.elstc.co/guide/en/search-ui/current/overview.html

Description

Adds AsciiDoc version of all MDX files.

List of changes

• Converts all MDX syntax to AsciiDoc syntax and saves it in a new AsciiDoc file. (I will delete the MDX files in a follow-up PR after this is merged and redirects are deployed.)
• Updates the index.asciidoc file to list all the pages in the same order/structure as defined in docs/nav-searchui.docnav.json.

Associated Github Issues

Related to #1093 elastic/docs#3140

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