[DOC] add DAC page for FORTIFY

[DanAlbert]

URL

new page

Problem

https://android-developers.googleblog.com/2017/04/fortify-in-android.html discusses this in detail and https://android.googlesource.com/platform/ndk/+/master/docs/BuildSystemMaintainers.md#fortify does briefly, but we ought to put this on DAC where it's more discoverable.

[enh-google]

(we should update bionic/docs/clang_fortify_anatomy.md to link to the new page too.)

[enh-google]

https://android.googlesource.com/platform/bionic/+/refs/heads/main/docs/clang_fortify_anatomy.md if you want a clickable link. (the bionic ELF TLS documentation is similar --- it starts with a "hey, this is documentation about how it's implemented ... if you're a user, try this other documentation instead".)