Documentation of /usr/src directory layout

Hello,

Currently, hier(7) contains detailed but outdated [1] documentation of 
the directory structure under /usr/src. The src tree's README.md file 
duplicates much of this information by maintaining a distinctly 
incomplete table.

I propose that we reduce the maintenance overhead by keeping one of 
these lists as the source of truth. README.md is the more natural option 
here: being located at the root of the src tree it is more discoverable 
(especially for those new to the src tree), and the raw markdown text is 
much more human-readable than mdoc. Of course, the added benefit is that 
README.md is presented front-and-center when browsing the git repository 
on GitHub, GitLab, etc. See it on my fork [2].

The list of /usr/src subdirectories will be removed from hier(7) and 
replaced with a sentence informing the reader they can consult README.md 
for the details.

I have retained and improved the list of directories under sys/ from 
hier(7), but to avoid polluting the top-level source roadmap I have 
added a secondary sys/README.md which documents the layout of the kernel 
sources.

I do not expect these changes to be contentious, but I'd like to cast a 
wider net for review. Productive feedback is welcome by email or 
directly in the Phabricator reviews:

https://reviews.freebsd.org/D37132
https://reviews.freebsd.org/D37133
https://reviews.freebsd.org/D37134
https://reviews.freebsd.org/D37135
https://reviews.freebsd.org/D37136

Also, this information is duplicated in at least one more place: the 
Developer's Handbook [3]. I intend to address this too after the current 
series of changes is complete. Most likely, this will be done by 
replacing the list with a link to the README.md file in cgit.

Thanks!
Mitchell

[1] https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=261349
[2] https://github.com/mhorne/freebsd/tree/hier
[3] 
https://docs.freebsd.org/en/books/developers-handbook/introduction/#introduction-layout