[feature] Add pre_validate post_validate hooks in reference (#4013)

* Add pre_validate post_validate hooks in reference

Signed-off-by: Uilian Ries <uilianries@gmail.com>

* Update reference/extensions/hooks.rst

* Update reference/extensions/hooks.rst

---------

Signed-off-by: Uilian Ries <uilianries@gmail.com>
Co-authored-by: Carlos Zoido <mrgalleta@gmail.com>

Author: uilianries

reference/extensions/hooks.rst

@@ -93,6 +93,12 @@ Here you can see a complete example of all the hook functions available:
     def post_export(conanfile):
         conanfile.output.info("Running after of executing export() method.")
 
+    def pre_validate(conanfile):
+        conanfile.output.info("Running before executing the validate() method.")
+
+    def post_validate(conanfile):
+        conanfile.output.info("Running after executing the validate() method.")
+
     def pre_source(conanfile):
         conanfile.output.info("Running before to execute source() method.")