theme.js: fix sidebar link finding code

[ffplexim]

Fixes a bug where the current section is not highlighted in the navigation sidebar if the current page's hash does not match the "primary anchor" (usually extracted from the section's title). For example, if the page's RST source has

.. _foo:

Bar
===

and you link to it with :ref:`foo` , "Bar" will not get selected in the sidebar.

This is because the code for selecting the current section in the sidebar is still assuming that section elements are of the form <div class="section">, instead of <section>. (docutils' HTML5 writer has been emitting the latter since docutils 0.17.)

[ffplexim]

The CI failures are unrelated; #1666 should fix them. Rebased after #1666 was merged.

[ffplexim]

Example of this bug occurring:

• https://docs.ansible.com/projects/ansible/devel/inventory_guide/intro_inventory.html#using-multiple-inventory-sources (following a link from somewhere else to an explicit label: section not selected in sidebar)
• https://docs.ansible.com/projects/ansible/devel/inventory_guide/intro_inventory.html#passing-multiple-inventory-sources (using title's link: section selected in sidebar)

CC @oraNod

[felixfontein]

@humitos @geographika is it possible to get this reviewed? Thanks :)

[felixfontein]

Ping @agjohnson