Add FuzzTest documentation.

This redirects our fuzzing documentation to advise use of FuzzTest
rather than libfuzzer.

Specifically, this is mostly a rename from
getting_started.md -> getting_started_with_libfuzzer.md

and then a whole new getting_started.md.

Bug: 1494445
Change-Id: Ic2c8064b87dea6276ee2d508af5396553809b332
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4970173
Reviewed-by: Paul Semel <[email protected]>
Commit-Queue: Adrian Taylor <[email protected]>
Cr-Commit-Position: refs/heads/main@{#1215199}
diff --git a/testing/libfuzzer/README.md b/testing/libfuzzer/README.md
index 29fc8af..8db1c7ef 100644
--- a/testing/libfuzzer/README.md
+++ b/testing/libfuzzer/README.md
@@ -30,6 +30,7 @@
 
 ## Advanced Topics
 
+* [Using libfuzzer instead of FuzzTest].
 * [Improving fuzz target efficiency].
 * [Creating a fuzz target that expects a protobuf] instead of a byte stream as
   input.
@@ -81,3 +82,4 @@
 [go/fuzzing-success]: https://goto.google.com/fuzzing-success
 [libFuzzer]: http://llvm.org/docs/LibFuzzer.html
 [libFuzzer integration]: libFuzzer_integration.md
+[Using libfuzzer instead of FuzzTest]: getting_started_with_libfuzzer.md