docs: Document CMAKE_BUILD_TYPE=Release for production builds

[sensei-hacker]

Summary

Adds documentation about using CMAKE_BUILD_TYPE=Release to save disk space when building firmware for production, CI, or release purposes.

Problem

By default, INAV builds with CMAKE_BUILD_TYPE=RelWithDebInfo which includes maximum debug symbols (-ggdb3). When building all ~100 targets, this consumes ~109 GB of disk space. The debug symbols are automatically stripped from the final .hex files, so they provide no benefit for production builds.

Solution

Document the use of CMAKE_BUILD_TYPE=Release for production builds, which reduces disk usage to ~4-6 GB (96% reduction) while producing identical final binaries.

Changes

docs/development/Building in Linux.md

• Added brief note about debug symbols in single target build section
• Added new "Building All Targets or Release Builds" section with:
  • Warning about 109 GB disk usage with default mode
  • Instructions for using Release mode
  • Disk usage comparison table

docs/development/release-create.md

• Added "Building Firmware Locally (if needed)" section with:
  • Release mode build commands
  • Disk usage comparison
  • Explanation that final binaries are identical

Key Points Documented

• Default (RelWithDebInfo): ~109 GB for all targets
• Release mode: ~4-6 GB for all targets (96% reduction)
• Command: cmake -DCMAKE_BUILD_TYPE=Release ..
• Use cases: Production builds, CI, building many targets
• Important: Final .hex and .bin files are identical in both modes

Also Includes

• Typo fix: "Bulding" → "Building"

Testing

Documentation changes only. No code modifications.