Fix unreachable pages under 3D Particles by using fake headers

[tetrapod00]

Closes #8792.

Applies a similar fix as #9989. Converts all headers in tutorials\3d\particles\index.rst to fake rubric headers. Note that in this case we can't have any real subheaders on the page at all, due to the order of the sections. If the page was structured like the following, the toctree would be counted as a subsection of Introduction and the pages would not appear in the sidebar.

Introduction
-------------

Some paragraphs of text.

.. rubric:: Basic usage

.. toctree::
   :maxdepth: 1
   :name: toc-particles-basic

   creating_a_3d_particle_system

Master	This PR

[tetrapod00]

If #10111 is approved, this will need changes.

[mhilbrunner]

Thanks!

[mhilbrunner]

Cherrypicked to 4.3 in #10346.