developers-handbook: Fix broken links

[macdonald.umoren_proton.me]

Summary

Fix links to Building and Installing a Custom Kernel
section of the handbook

Test Plan

Links in generated html are changed from https://docs.freebsd.org/en/books/handbook/#kernelconfig-building to https://docs.freebsd.org/en/books/handbook/kernelconfig/#kernelconfig-building

[fernape]

Change points to

https://docs.freebsd.org/en/books/handbook/#kernelconfig-building but I think the proper anchor is:
https://docs.freebsd.org/en/books/handbook/book/#kernelconfig-building

Otherwise we end up in the chapter-splitted version of the handbook when clicking on the link and we don't go to the kernel config specific section.

[macdonald.umoren_proton.me]

In D40287#917802, @fernape wrote:

Change points to

https://docs.freebsd.org/en/books/handbook/#kernelconfig-building but I think the proper anchor is:
https://docs.freebsd.org/en/books/handbook/book/#kernelconfig-building

The changes point to https://docs.freebsd.org/en/books/handbook/kernelconfig/#kernelconfig-building

[fernape]

In D40287#918020, @macdonald.umoren_proton.me wrote:

In D40287#917802, @fernape wrote:

Change points to

https://docs.freebsd.org/en/books/handbook/#kernelconfig-building but I think the proper anchor is:
https://docs.freebsd.org/en/books/handbook/book/#kernelconfig-building

The changes point to https://docs.freebsd.org/en/books/handbook/kernelconfig/#kernelconfig-building

I just rebuilt the doc again and I see you're right. Sorry for the noise!

[fernape]

LGTM!

[macdonald.umoren_proton.me]

Thanks for the review. Please go ahead and commit