Add float_bits_conv to unstable book

Author: est31

src/doc/unstable-book/src/SUMMARY.md

@@ -72,6 +72,7 @@
 - [fd](fd.md)
 - [fd_read](fd-read.md)
 - [fixed_size_array](fixed-size-array.md)
+- [float_bits_conv](float-bits-conv.md)
 - [float_extras](float-extras.md)
 - [flt2dec](flt2dec.md)
 - [fmt_flags_align](fmt-flags-align.md)

src/doc/unstable-book/src/float-bits-conv.md

@@ -0,0 +1,7 @@
+# `float_bits_conv`
+
+The tracking issue for this feature is: [#40470]
+
+[#40470]: https://github.com/rust-lang/rust/issues/40470
+
+------------------------