MAINTAINERS: update btrfs website links and files

We have the new documentation hosted on Read The Docs and content is
migrated there from the wiki. Also update http to https and add the
tracepoint definition header.
Signed-off-by: David Sterba <dsterba@suse.com>

MAINTAINERS

@@ -4428,13 +4428,15 @@ M:	Josef Bacik <josef@toxicpanda.com>
 M:	David Sterba <dsterba@suse.com>
 L:	linux-btrfs@vger.kernel.org
 S:	Maintained
-W:	http://btrfs.wiki.kernel.org/
-Q:	http://patchwork.kernel.org/project/linux-btrfs/list/
+W:	https://btrfs.readthedocs.io
+W:	https://btrfs.wiki.kernel.org/
+Q:	https://patchwork.kernel.org/project/linux-btrfs/list/
 C:	irc://irc.libera.chat/btrfs
 T:	git git://git.kernel.org/pub/scm/linux/kernel/git/kdave/linux.git
 F:	Documentation/filesystems/btrfs.rst
 F:	fs/btrfs/
 F:	include/linux/btrfs*
+F:	include/trace/events/btrfs.h
 F:	include/uapi/linux/btrfs*
 
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