Explain better how to add data files to a sample

Also don't duplicate this information in both how-to-add-new-sample.md
and how-to-add-files-to-build-system.md, just reference the former in
the latter instead.

Also fix some Markdown markup.

docs/contributing/how-to-add-files-to-build-system.md

@@ -120,24 +120,11 @@ wxUniversal, e.g. GDI classes. Files shared by all X Window System ports
 should be put into `XWIN_LOWLEVEL_SRC`.
 
 
-Adding sample
--------------
+Adding a new sample
+-------------------
 
-Copy the bakefile from another sample, change the ID and files accordingly.
-If the sample uses some data files, make sure to have `<wx-data>` node
-in the sample's bakefile (see e.g. `samples/image/image.bkl` for an example).
-Make sure to add `<wx-lib>` statements for all libraries from multilib build
-that are required by the sample.
-
-The Windows resource specification should use the central .rc file:
-
-        <win32-res>../sample.rc</win32-res>
-
-Run `bakefile_gen` in `$(wx)/build/bakefiles` to regenerate the bakefiles.
-
-Finally commit `$(wx)/build/bakefiles/make_dist.mk` and all the other modified files.
-
-Currently we commit all the generated makefiles.
+This is explained in details in `how-to-add-new-sample.md` file, please see
+there.
 
 
 Adding new core library

docs/contributing/how-to-add-new-sample.md

@@ -11,30 +11,36 @@ samples/ with demos/ where needed).
 
    Note that unless your sample requires something special you shouldn't
    add foo.rc, foo.ico, ... any more, please just reuse the generic `sample.*`
-   files in the samples subdirectory.
+   files in the samples subdirectory. In particular, the Windows resource
+   specification should use the central `.rc` file, i.e. do not use
+   `<win32-res>` bakefile tag unless it is really necessary.
 
 3. Create the makefiles:
    - modify samples/samples.bkl (just copy an existing line)
-   - create foo.bkl (`sed 's/minimal/foo/g' minimal.bkl > foo.bkl` is usually
+   - create `foo.bkl`, typically by just copying an existing bakefile from
+   another sample (`sed 's/minimal/foo/g' minimal.bkl > foo.bkl` is usually
    enough, but you may need to add more `<wx-lib>` lines if your sample uses
    anything not in the core lib)
+   - add `<wx-data>` element with the data files used by the sample that need
+   to be copied to its build directory, if any (see e.g.
+   `samples/image/image.bkl` for an example of using this tag).
    - generate the makefiles for your sample using bakefile. For this you
    need to:
-      - install bakefile (see docs/contributing/how-to-add-files-to-build-system.md)
-      - run bakefile_gen in build/bakefiles which will regenerate all
+      - install bakefile (see `how-to-add-files-to-build-system.md`)
+      - run `bakefile_gen` in `build/bakefiles` which will regenerate all
         makefiles which are not up to date
 
       You may also use `bakefile_gen -b ../../samples/foo/foo.bkl` to
       regenerate only the makefiles for this sample (path must be relative!)
       or even add a `-f<compiler>` to generate just the makefiles for the given
       compiler (run `bakefile --help` to get the list of possible values).
-      See how-to-add-files-to-build-system.md for more information.
+      Again, see `how-to-add-files-to-build-system.md` for more information.
 
 4. Modify configure.in Unix compilation:
    - if the sample should only be built if `wxUSE_FOO` is enabled, locate
       the test for `wxUSE_FOO = yes` in configure.in and add a line
       `SAMPLES_SUBDIRS="$SAMPLES_SUBDIRS foo"` under it
-   b) if it should be always built, locate the line `if test $wxUSE_GUI = yes`
+   - if it should be always built, locate the line `if test $wxUSE_GUI = yes`
      near the end of configure.in and modify the assignment to
      `SAMPLES_SUBDIRS` to include "foo" (put in alphabetical order)
 
@@ -46,15 +52,17 @@ samples/ with demos/ where needed).
 
         wx_add_sample(foo DEPENDS wxUSE_FOO)
 
+   Use `DATA` CMake variable if the sample uses any data files.
+
    For a complete list of parameters to the `wx_add_sample()` function see
    the description in `build/cmake/functions.cmake`.
 
 6. Add a short description of what the sample does and how does it work
    to the "samples overview" section in the wxWidgets manual. That section
-   lives in docs/doxygen/mainpages/samples.h; look at the descriptions for
+   lives in `docs/doxygen/mainpages/samples.h`; look at the descriptions for
    other samples, if you are not familiar with Doxygen.
 
 7. Add any non-standard sample's files to `build/bakefiles/make_dist.mk` (the
    makefiles copies all bmp, cpp, h, ico, png, rc, xpm and makefiles by
    default, you only need to update it if the sample uses some other files)
-   and run the ./update-manifests.sh script in distrib/scripts.
+   and run the `./update-manifests.sh` script in distrib/scripts.