Add merged best practices to "Best Practices for CNF Developers" doc

[Smitholi67]

The following best practices have already been merged:

https://github.com/lfn-cnti/bestpractices/blob/main/doc/cbpps/0001-cnf-best-practice-proposal-process.md
https://github.com/lfn-cnti/bestpractices/blob/main/doc/cbpps/0003-exceptions.md
https://github.com/lfn-cnti/bestpractices/blob/main/doc/cbpps/0004-do-not-run-containers-with-privilege-flag.md
https://github.com/lfn-cnti/bestpractices/blob/main/doc/cbpps/0005-single-concern-per-container.md

However, they are not linked on the Best Practices for CNF Developers doc: https://github.com/lfn-cnti/bestpractices/blob/main/doc/best_cnf_dev.md.

Please add these links to the appropriate sections of https://github.com/lfn-cnti/bestpractices/blob/main/doc/best_cnf_dev.md.

[lixuna]

Refs: #271

[lixuna]

https://github.com/lfn-cnti/bestpractices/blob/main/doc/cbpps/0001-cnf-best-practice-proposal-process.md is a template to describe how to create a best practice - so I will not add it to the list of best practices.

[lixuna]

https://github.com/lfn-cnti/bestpractices/blob/main/doc/cbpps/0003-exceptions.md is a template to describe how to "Document exceptions to following best practices and community standards" - so I will not add it to the list of best practices

[lixuna]

https://github.com/lfn-cnti/bestpractices/blob/main/doc/cbpps/0004-do-not-run-containers-with-privilege-flag.md is already listed on https://github.com/lfn-cnti/bestpractices/blob/main/doc/best_cnf_dev.md

[lixuna]

Added https://github.com/lfn-cnti/bestpractices/blob/main/doc/cbpps/0005-single-concern-per-container.md in PR #351