doc: MyFirstContribution: fix missing dependencies and clarify build steps

doc: MyFirstContribution: fix missing dependencies and clarify build steps

Fix issues in the MyFirstContribution guide that can lead to confusion or
test failures when following the documented steps.

* Add missing header includes in code examples (environment.h and strbuf.h).

* Correct manpage synopsis formatting to prevent failing documentation tests.

* Specify the use of parallel test execution with -j$(nproc), noting that it
runs tests using all available CPUs and may be adjusted.

These updates improve documentation accuracy and make the first-time contributor
journey smoother.

Signed-off-by: Shreyansh Paliwal <shreyanshpaliwalcmsmn@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>