Documentation Enhancement: Add build2 to the list of supported build systems and package managers

[lyrahgames]

Description

build2 is a growing C/C++ build toolchain that already supports the importation of doctest via its official package repository and GitHub.
Thus, it would be beneficial to also add this information to the documentation in doc/markdown/build-systems.md together with its basic usage, like it was done here.
Additionally, I would consider a usage example in examples/doctest_build2/ as it was done for CMake in examples/installed_doctest_cmake/ to be very helpful.
Also, I would offer to add the stated information to the documentation myself by a pull request. Referring to the contribution guidelines, this issue is only meant to get the confirmation and make this contribution go smoothly.
Thank you all so much for your hard work!