doc: update 5.2.0 release (#46)

* DOC/MAJOR: some corrections, update assets, additions for compounds and templates related topics and

* DOC: user-guide: update descriptions, add some assets.

- compound editor help guide
- doc-info.png : update with current setup (Addons, Import/Export) screenshot
- restore an entry : update documentation and asset with description
- tinymce mathjax section: lots of exp from the populate default within <pre> tags, so show how to remove it.
- user-guide.rst : replace image:: tags with figure:: and add description. Harmonize -> all pictures centered and not just some of them, all with descripions. Missing gifs but will come later on! (e.g. Tables gif, tags..). Ref to Addons section, update "restore entry"

* DOC: some adjustments

* images description

Author: MoustaphaCamara

doc/addons.rst

@@ -71,3 +71,5 @@ Then enable it in ``elabftw`` container configuration:
     # This is for the integration of the DNA Cloning tool
     - USE_OPENCLONING=true
     - OPENCLONING_URL=http://opencloning-plugin:8000/
+
+Restart the ``elabftw`` container to take these changes into account.