Thelio Mira R4-N4 (initial) & transparent image bg for dark themes

[jacobgkau]

This adds the Thelio Mira R4-N4 (thelio-mira-r4-n4) to tech docs.

[jacobgkau]

This is ready for review for initial release. This should be rebased & merged (not squashed) when ready.

I will complete the following items in a follow-up PR next week:

• More photos for installing 2.5" drives.
  • I'd put this part off since we were unclear where the 2.5" drive screws were going to end up shipping, but I came up with a way to word it that covers all possible cases.
• Steps to install the optional additional 2.5" or 3.5" drive cage.
  • A cover for the 3.5" drive cage was not available for photos this week (R&D tossed them all to make sure no prototypes would accidentally ship), but we should apparently have some in the next week or so.
• Steps to replace the front I/O board and power button.
  • Low-priority sections due to not being something needed super often (especially with the power button no longer being on a separate top case).
• Steps to replace the fans in the left side panel, along with more images & detailed steps for replacing the bottom fans.
  • Just didn't get to this one yet.
• Will also add top & bottom photos to the external overview.
  • Marketing didn't have head-on angles taken, so I'll take them.

A note is currently present on the Repairs page advising that it's not fully complete. (These note blocks are a new mdbook feature we got with #341.)

Finally, I was also able to address #206. External Overview images (with text) will now have a white background in the dark themes, while still being fully transparent to look nice in the Rust light theme. The first commit here adds the CSS for this and applies it to all existing sections that needed it.

[leviport]

I have a few things (mostly related to radiator/heatsink naming), but this otherwise looks great!

[leviport]

🤘