Fix dead link fluidhowto mercurial

Author: paugier

CONTRIBUTING.md

@@ -23,4 +23,4 @@ are hosted here: <https://foss.heptapod.net/fluiddyn>.
 Please read our documentation on
 [developer essentials](https://fluiddyn.readthedocs.io/en/latest/advice_developers.html),
 and especially on
-[setting up Mercurial](https://fluidhowto.readthedocs.org/en/latest/mercurial.html).
+[setting up Mercurial](https://fluidhowto.readthedocs.org/en/latest/mercurial/index.html).

doc/build-from-source.md

@@ -19,7 +19,7 @@ Mercurial extensions like [Evolve] and [Topic]. The main repository is hosted
 
 There are other ways to get the source but we are going here to assume that you can
 install [Mercurial]. It can be useful when working with Fluidimage source to
-[fully setup Mercurial with these extensions and learn a bit of Mercurial](https://fluidhowto.readthedocs.org/en/latest/mercurial.html).
+[fully setup Mercurial with these extensions and learn a bit of Mercurial](https://fluidhowto.readthedocs.org/en/latest/mercurial/index.html).
 Then, the Fluidimage repository can be cloned with
 
 ```sh