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ARROW-15349: [Doc][Guide] Existing Contributors page - update

This PR is a proposal to rearrange the existing [contributing page](https://arrow.apache.org/docs/developers/contributing.html) to serve as a landing page for topics connected to contribution to Apache Arrow. Main topics are as follows:

- Community
- New Contributor's Guide
- Advanced code contribution
- Report bugs and propose features

The existing content was transferred to two new pages (_bug_reports.rst_, _advanced.rst_) and to existing section of the New Contributor's Guide.

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**Working html version is available on this link:**
https://alenkaf.github.io/arrow-dev-docs-preview/html/developers/contributing.html

***

TODO:

- [x] Convert `contributing.rst` to a landing page with panels to link the content.
- [x] _Code of Conduct and Mailing List_ - link to [existing Arrow page](https://arrow.apache.org/community/) and add a note about ML.
- [x] _Report bugs and propose features_ moves to it's own page (_bug_reports.rst_).
- [x] _Improve documentation_ - becomes a dropdown on the landing site:
  -   add link from the landing page to the Guide section for documentation,
  -   add link to the building of the documentation.
- [x] _Contribute code_ moves to the Guide.
- [x] _Experimental repositories and Guidance for specific features_ would live in _Advanced code contribution_ where the links to the existing docs would be added (_advanced.rst_).
- [x] Move all the pictures into `developers/images`.
- [x] Add example reprex for R and Python.
- [x] Add _Report bugs and propose features_ to the sidebar.
- [x] Use `sphinx-design` instead of `sphinx-panels`.
- [x] Try to form the info on the _Advanced code contribution_  page into a checklist.
- [x] Reshuffle cards on the landing page to have _Report bugs and propose features_ in the first line and _New Contributors Guide_ on the second.
- [x] Update https://github.com/apache/arrow/blob/master/CONTRIBUTING.md to be in sync with `contributing.rst`.

Closes #12483 from AlenkaF/ARROW-15349

Lead-authored-by: Alenka Frim <frim.alenka@gmail.com>
Co-authored-by: Alenka Frim <AlenkaF@users.noreply.github.com>
Co-authored-by: Nicola Crane <thisisnic@gmail.com>
Signed-off-by: Joris Van den Bossche <jorisvandenbossche@gmail.com>