Partially revert "ReadMe & docs: The Haiku Book has moved to www.haik…

…u-os.org/docs/api."

This mostly reverts commit 75b219d.
The changes to the image URLs in Alert.dox are still needed, so I
didn't revert those.

As per discussion on the mailing list and IRC.

ReadMe.md

@@ -4,7 +4,7 @@ Haiku
 | [Mailing Lists](https://www.haiku-os.org/community/ml)
 | [IRC Channels](https://www.haiku-os.org/community/irc)
 | [Issue Tracker](https://dev.haiku-os.org/)
-| [API docs](https://www.haiku-os.org/docs/api/)**
+| [API docs](https://api.haiku-os.org)**
 
 Haiku is an open-source operating system that specifically targets personal
 computing. Inspired by the BeOS, Haiku is fast, simple to use, easy to learn

docs/user/book.dox

@@ -5,7 +5,7 @@
 	Interface (API) of the Haiku operating system. This API describes
 	the internals of the operating system allowing developers to write
 	native C++ applications and device drivers. See the
-	<a href="https://www.haiku-os.org/docs/api/">online version</a> for the most
+	<a href="https://api.haiku-os.org">online version</a> for the most
 	updated version of this document. If you would like to help contribute
 	contact the <a href="https://www.freelists.org/list/haiku-doc">documentation
 	mailing list</a>. For guidelines on how to help document the API see
@@ -20,7 +20,7 @@
 	appropriate sections.
 
 	A complete reference to the BeOS R5 API is available on the web in
-	<a href="https://www.haiku-os.org/legacy-docs/bebook/">The Be Book</a>.
+	<a href="https://haiku-os.org/legacy-docs/bebook/">The Be Book</a>.
 	The Be Book is used with permission from
 	<a href="https://www.access-company.com/">Access Co.</a>, the current
 	owners of Be's intellectual property.