[v1.16] Add relnote etc

[hamishwillee]

This updates the release note and release sidebar for v1.16 branch.

With some luck this will build us a v1.16 release doc.

The flaws in the release note doc are real - they exist because some things were added as backports without the docs. We can fix this "a bit" by adding dshot, but I propose to do that later.

[github-actions]

/en/releases/1.16.md

• LinkedFileMissingAnchor: #bidirectional-dshot-telemetry not found in ../peripherals/dshot.md (/home/runner/work/PX4-Autopilot/PX4-Autopilot/docs/en/peripherals/dshot.md)
• LinkedFileMissingAnchor: #bidirectional-dshot-telemetry not found in ../peripherals/dshot.md (/home/runner/work/PX4-Autopilot/PX4-Autopilot/docs/en/peripherals/dshot.md)
• LinkedFileMissingAnchor: #ackermann not found in ../frames_rover/index.md (/home/runner/work/PX4-Autopilot/PX4-Autopilot/docs/en/frames_rover/index.md)
• LinkedFileMissingAnchor: #differential not found in ../frames_rover/index.md (/home/runner/work/PX4-Autopilot/PX4-Autopilot/docs/en/frames_rover/index.md)
• LinkedFileMissingAnchor: #mecanum not found in ../frames_rover/index.md (/home/runner/work/PX4-Autopilot/PX4-Autopilot/docs/en/frames_rover/index.md)
• LinkedInternalPageMissing: This linked file is missing: ../config_rover/index.md
• LinkedInternalPageMissing: This linked file is missing: ../flight_modes_rover/manual.md
• LinkedInternalPageMissing: This linked file is missing: ../flight_modes_rover/manual.md
• LinkedInternalPageMissing: This linked file is missing: ../flight_modes_rover/manual.md
• LinkedInternalPageMissing: This linked file is missing: ../flight_modes_rover/manual.md
• LinkedInternalPageMissing: This linked file is missing: ../flight_modes_rover/auto.md
• LinkedInternalPageMissing: This linked file is missing: ../flight_modes_rover/manual.md
• LinkedInternalPageMissing: This linked file is missing: ../flight_modes_rover/manual.md
• LinkedInternalPageMissing: This linked file is missing: ../flight_modes_rover/manual.md
• LinkedInternalPageMissing: This linked file is missing: ../flight_modes_rover/manual.md
• LinkedInternalPageMissing: This linked file is missing: ../flight_modes_rover/auto.md
• LinkedInternalPageMissing: This linked file is missing: ../flight_modes_rover/manual.md
• LinkedInternalPageMissing: This linked file is missing: ../flight_modes_rover/manual.md
• LinkedInternalPageMissing: This linked file is missing: ../flight_modes_rover/manual.md
• LinkedInternalPageMissing: This linked file is missing: ../flight_modes_rover/manual.md
• LinkedInternalPageMissing: This linked file is missing: ../flight_modes_rover/auto.md
• LinkedInternalPageMissing: This linked file is missing: ../config_rover/position_tuning.md
• PageNotInTOC: Page not in Table of Contents (/home/runner/work/PX4-Autopilot/PX4-Autopilot/docs/en/SUMMARY.md)

/en/SUMMARY.md

• UrlToLocalSite: Link is URL to this site. Should it be relative link?: [main (alpha)](https://docs.px4.io/main/en/releases/main.html))

/en/releases/index.md

• UrlToLocalSite: Link is URL to this site. Should it be relative link?: [main](https://docs.px4.io/main/en/releases/main.html))

/en/releases/main.md

• PageNotInTOC: Page not in Table of Contents (/home/runner/work/PX4-Autopilot/PX4-Autopilot/docs/en/SUMMARY.md)
• UrlToLocalSite: Link is URL to this site. Should it be relative link?: [See the latest version here.](https://docs.px4.io/main/en/releases/main.html))