Update documentation for clang-format

[mikee47]

This PR updates the Sming coding style documentation as discussed in #2898 (comment) and #2904 (comment).

Sming requires version 8 which is generally no longer available in the standard repositories for recent GNU/Linux distributions. Different versions of clang-format produce different output with the same configuration. This is such a common problem that a kind soul has provided standalone builds here https://github.com/muttleyxd/clang-tools-static-binaries/releases.

The default name of the clang-format executable has been changed to clang-format-8. This is because clang-format is now very unlikely to be the default installed version, and so avoids the subtle issues with running the wrong version.

[what-the-diff]

PR Summary

• Improved the installation process
  This pull request includes detailed instructions for installing clang-format, making the process easier for users. This includes a specific workaround to handle any issues with obtaining version 8.
• Provided examples for setup
  We provided a set of example commands that help users set up clang-format from standalone binaries.
• Clarified the usage of CLANG_FORMAT
  We've elaborated on why you need to save the CLANG_FORMAT environment variable for Sming's operation. The default has been changed from clang-format to clang-format-8.
• Enhanced content arrangement
  The markdown headings have been rearranged to improve content structure. The clang-format section has been moved to its own header for better organization.
• Facilitated clang-tidy installation
  The PR has added instructions for the installation of clang-tidy on Ubuntu, offering increased clarity on how to implement this install.
• Updated specification for supported versions
  The PR updates the note detailing the recommended clang-tidy version. It now encourages the use of the most recent version for optimized performance.