Reorganize software guide

[cjsha]

• Add sphinx-design
• Add cards on front page
• Delete extraneous page
• GUI card & TOC entry will link to an external page once that page exists in the gui-docs site
• Cards still need images

[ChucklesOnGitHub]

I've taken some poetic license here, but bear with me. You had tied each button to the application instead of the package/plugin and I think this works really nicely for users, as opposed to using the name of the package or plugin. We can go into the details in the corresponding pages. I tried to keep the same hierarchical structure for everything: mention application then package/plugin , so they may not be 100% gramatically correct. I also avoided colons in the card titles.

In line with this, I've referenced the application in the navigation because Bonsai package is more relatable than its formal name.
I also titled the section about the old library Deprecated Software, following the title it had in the section landing page, to avoid the "Bonsai" in Bonsai.ONIX to jump out. I think this works nicely.

GUI card & TOC entry will link to an external page once that page exists in the gui-docs site

While this would keep the click-out hierarchy the same as Bonsai so for me it makes sense, I think the content from the current index.rst in the GUI plugin section should be reflected in the Software Guide section, because the two applications will not provide the same system functionality (OE GUI is not for imaging data, not for stimulation, etc - or is it).

Cards still need images

I think referring to the package/plugin by the application logo works.

[ChucklesOnGitHub]

@cjsha take a look

[cjsha]

@jonnew This PR is supposed to make navigating to the software docs from the hardware docs easier. I tagged you for review because I know you worked on this previously and so I don't want to change these pages without your acknowledgement.

The first card and first toc entry link to bonsai-onix1-docs
The second card and second toc entry link to this placeholder page until there is a page for the onix-source plugin in the gui-docs