Improve reference for the build_id() method

[danimtb]

After a question around the build_id() method at conan-io/conan#3106 (comment), I was reviewing this section of the documentation and found that it is not properly explained and in some cases the concept of the build ID can lead to confusion, so I decided to improve it.

Main additions:

• Better structure.
• Reword the use cases
• Explain the default behavior of the build folder and how the build_id() modifies this.
• Better illustrate the example.
• Add the conan commands needed to create the package (and reuse the build folder).
• Improve the explanation of the conditional build ID.

[memsharded]

Good job