Revert "migrate docs to mdx"

This reverts commit 3cb12b5a52.

website/docs/language/functions/tostring.mdx

@@ -1,33 +0,0 @@
----
-page_title: tostring - Functions - Configuration Language
-description: The tostring function converts a value to a string.
-sidebar_title: tostring
----
-
-# `tostring` Function
-
-`tostring` converts its argument to a string value.
-
-Explicit type conversions are rarely necessary in Terraform because it will
-convert types automatically where required. Use the explicit type conversion
-functions only to normalize types returned in module outputs.
-
-Only the primitive types (string, number, and bool) and `null` can be converted to string.
-All other values will produce an error.
-
-## Examples
-
-```
-> tostring("hello")
-hello
-> tostring(1)
-1
-> tostring(true)
-true
-> tostring(null)
-null
-> tostring([])
-Error: Invalid function argument
-
-Invalid value for "v" parameter: cannot convert tuple to string.
-```