Close/redirect all remaining old/duplicate pages from `gh-pages`

[ES-Alexander]

Continuing in the vein of #187, there are multiple old/duplicate pages hanging around that don't get updated with their new counterparts. Listing and checking them off here.

• [x] troubleshooting/troubleshooting.md -> redirect to reference/troubleshooting.html
• [ ] software/mavlink.md
• [ ] hardware/components.md
• [ ] operators-manual/outputs.md
• [ ] software/ardusub-firmware.md
• [ ] images/reference/reference.md
• [ ] reference/companion/web-ui.md
• [ ] getting-started/installation.md
• [ ] getting-started/initial-setup.md
• [ ] images/quick-start/quick-start.md
• [ ] reference/ardusub/firmware-page.md
• [ ] reference/features-while-in-operation.md
• [ ] getting-started/installation-MDONLY.md

[ES-Alexander]

This is an extension of #136 (which I just saw/found).