Update release guide with lessons from 9.1.0-RC1

[sensei-hacker]

Summary

Updates docs/development/release-create.md with lessons learned during the 9.1.0-RC1 release process.

• Version string format (new section): RC filenames must use lowercase rc with a hyphen (e.g., 9.1.0-rc1). The Configurator firmware flasher uses a case-sensitive regex; uppercase RC or underscore separators cause the target board name to be misread, making the firmware invisible in the flasher even after a successful upload.
• Fix rename script: The output filename used _rc2_ (underscore) but the required format is -rc2_ (hyphen). This would have produced filenames that the Configurator cannot parse.
• Fix configurator artifact download: The previous instructions used find . -mindepth 2 -type f -exec mv -t . {} + to flatten all files into one directory. This exact pattern caused a Windows .exe to be packaged inside a macOS DMG in the 9.0.0 release. Replaced with explicit per-platform mkdir/mv.
• PR branch verification (changelog section): gh pr list returns PRs by merge date regardless of target branch, so PRs merged to maintenance-10.x appear in results even if they are not part of the current release. Added commands to verify each PR is actually on the release branch.
• gh release create examples: Added --prerelease for RC releases and --target <commit-sha> to tag a specific commit atomically without needing local repo access.
• Post-publish verification: Added a step to verify firmware appears in the Configurator Firmware Flasher tab (with "Show unstable releases" enabled) before publishing the Configurator release. This is the end-to-end validation that the filename format and release structure are correct.

Test plan

• Read through the updated document for consistency
• Verify the rename script example produces inav_9.1.0-rc1_TARGET.hex format
• Confirm no other references to the old uppercase RC naming remain

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[github-actions]

Branch Targeting Suggestion

You've targeted the master branch with this PR. Please consider if a version branch might be more appropriate:

• maintenance-9.x - If your change is backward-compatible and won't create compatibility issues between INAV firmware and Configurator 9.x versions. This will allow your PR to be included in the next 9.x release.

• maintenance-10.x - If your change introduces compatibility requirements between firmware and configurator that would break 9.x compatibility. This is for PRs which will be included in INAV 10.x

If master is the correct target for this change, no action is needed.

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This is an automated suggestion to help route contributions to the appropriate branch.

[github-actions]

Test firmware build ready — commit 289dd88

Download firmware for PR #11621

238 targets built. Find your board's .hex file by name on that page (e.g. MATEKF405SE.hex). Files are individually downloadable — no GitHub login required.

Development build for testing only. Use Full Chip Erase when flashing.