minor #4200 Moved 'contributing' images to their own directory (javiereguiluz)

This PR was merged into the 2.3 branch.

Discussion
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Moved 'contributing' images to their own directory

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | 2.3+
| Fixed tickets | #3192

The PDF version of the "Contributing Guide" doesn't show the images (as reported in #3192). The rest of the PDFs correctly show them. Although I don't know the exact cause of the error, the "Contributing" guide images weren't stored in their own subdirectory inside `images/` and named after the book. That's the only difference between the right and the wrong PDF files.

For compatibility reasons I've left on purpose the original images at their location and just made a copy in the new directory.

Commits
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fc15b65 Moved 'contributing' images to their own directory

Author: weaverryan

contributing/community/releases.rst

@@ -77,7 +77,7 @@ Schedule
 
 Below is the schedule for the first few versions that use this release model:
 
-.. image:: /images/release-process.jpg
+.. image:: /images/contributing/release-process.jpg
    :align: center
 
 * **Yellow** represents the Development phase

contributing/documentation/overview.rst

@@ -61,7 +61,7 @@ If you have made your changes based on the 2.3 branch then you need to change
 the base branch to be 2.3 on the preview page by clicking the ``edit`` button
 on the top left:
 
-.. image:: /images/docs-pull-request-change-base.png
+.. image:: /images/contributing/docs-pull-request-change-base.png
    :align: center
 
 .. note::