Add new encoding keyword for configure_file to manual

Also add a section how to deal with file encodings.

docs/markdown/Configuration.md

@@ -113,6 +113,14 @@ Will produce:
 #define BAR
 ```
 
+## Dealing with file encodings
+
+The default meson file encoding to configure files is utf-8. If you need to
+configure a file that is not utf-8 encoded the encoding keyword will allow
+you to specify which file encoding to use. It is however strongly advised to
+convert your non utf-8 file to utf-8 whenever possible. Supported file
+encodings are those of python3, see [standard-encodings](https://docs.python.org/3/library/codecs.html#standard-encodings).
+
 # A full example
 
 Generating and using a configuration file requires the following steps:

docs/markdown/Reference-manual.md

@@ -214,6 +214,9 @@ the `@variable@` syntax.
   was specified. It defaults to `c`, in which case preprocessor directives
   will be prefixed with `#`, you can also use `nasm`, in which case the
   prefix will be `%`.
+- `encoding` *(added v0.47.0)* set the file encoding for the input and output file,
+  defaults to utf-8. The supported encodings are those of python3, see
+  [standard-encodings](https://docs.python.org/3/library/codecs.html#standard-encodings).
 
 ### custom_target()