👌 Improve cross-theme compatibility of need collapse button

[chrisjsewell]

Currently, the need collapse toggle icons are added to the page like:

Wrapping the SVG icons in an <img src=...> is not great, because it does not allow it to be properly styled.
Indeed for our own docs, we have to use the horrible hack of filter: brightness(0) invert(1), to get these icons to have the correct color for light/dark mode 😬

This PR instead places the entire SVG onto the page, and adds some core CSS to set it at the correct relative size (and float right)

the SVG then matches the local text color, without any need for additional CSS:

Also, the initial visibility of the icons is now set correctly, before any JS is run,
so that you don't get a bad render experience, whereby both icons are initial visible (until the JS is triggered)

Note, it might be ideal to de-duplicate these SVGs on the page (see e.g. this stackoverflow), but given these icons are small I don't think this is worth the added complexity for now

---

part of addressing #1172

[codecov]

Codecov Report

Attention: Patch coverage is 85.18519% with 4 lines in your changes are missing coverage. Please review.

Project coverage is 86.21%. Comparing base (dd28cda) to head (4b3eb73).

Files	Patch %	Lines
sphinx_needs/layout.py	85.18%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1181      +/-   ##
==========================================
- Coverage   86.31%   86.21%   -0.10%     
==========================================
  Files          56       56              
  Lines        6494     6498       +4     
==========================================
- Hits         5605     5602       -3     
- Misses        889      896       +7     

Flag	Coverage Δ
pytests	86.21% <85.18%> (-0.10%)	⬇️

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[chrisjsewell]

Another note, https://sphinx-togglebutton.readthedocs.io also directly adds the SVG to the page, they also wrap it in a button rather than a span, which may be better for accessibility, but I won't do that in this PR
https://github.com/executablebooks/sphinx-togglebutton/blob/f468ca6c5fd4caf0085a97f84794d733794f3614/sphinx_togglebutton/_static/togglebutton.js#L28