[Nexthop] Document build type options

[anna-nexthop]

Pre-submission checklist

• I've ran the linters locally and fixed lint errors related to the files I modified in this PR. You can install the linters by running pip install -r requirements-dev.txt && pre-commit install
• pre-commit run

Summary

Document the --build-type option for the getdeps.py build script and refactor the argument definition to eliminate code duplication.

• docs/docs/build/Building_FBOSS_on_containers.md: Added comprehensive documentation about build types (Debug, Release, RelWithDebInfo, MinSizeRel) with a table explaining each option

• docs/docs/platform/Building_Platform_Services.md: Updated build commands to use --build-type flag

• docs/static/code_snips/*.sh: Updated all build script examples to use --build-type instead of passing CMAKE_BUILD_TYPE in --extra-cmake-defines

• Updated the --build-type help text in getdeps.py to include:

  • Concise descriptions of each build type
  • Indication that RelWithDebInfo is the default
  • Removed the outdated "widely supported" comment

• Created a shared BUILD_TYPE_ARG kwargs dictionary to define the --build-type argument once

• Replaced three duplicate argument definitions in BuildCmd, TestCmd, and GenerateGithubActionsCmd classes

• Reduced code duplication and improved maintainability

• Users now understand what each build type does

• Cleaner syntax: --build-type MinSizeRel vs JSON in --extra-cmake-defines

• Single source of truth for the argument definition

• Easier to maintain and update in the future

Verified that help output works correctly for all three commands:

• build --help
• test --help
• generate-github-actions --help

Test Plan

Visually inspected a local serve of the documentation