move installation logic to the build script where it belongs

* build.zig: introduce `-Dflat` option which makes the installation
  match what we want to ship for our download tarballs. This allows
  deleting a bunch of shell script logic from the CI.
  - for example it puts the executable directly in prefix/zig rather
    than prefix/bin/zig and it additionally includes prefix/LICENSE.
* build.zig: by default also install std lib documentation to doc/std/
  - this can be disabled by `-Dno-autodocs` similar to how there is
    already `-Dno-langref`.
* build.zig: add `std-docs` and `langref` steps which build and install
  the std lib autodocs and langref to prefix/doc/std and
  prefix/doc/langref.html, respectively.

* std.Build: implement proper handling of `-femit-docs` using the
  LazyPath system. This is a breaking change.
  - this is a partial implementation of #16351
* frontend: fixed the handling of Autodocs with regards to caching and
  putting the artifacts in the proper location to integrate with the
  build system.
  - closes #15864

* CI: delete the logic for autodocs since it is now handled by build.zig
  and is enabled by default.
  - in the future we should strive to have nearly all the CI shell
    script logic deleted in favor of `zig build` commands.
* CI: pass `-DZIG_NO_LIB=ON`/`-Dno-lib` except for the one command where
  we want to actually generate the langref and autodocs. Generating the
  langref takes 14 minutes right now (why?!) so we don't want to do that
  more times than necessary.

* Autodoc: fixed use of a global variable. It works fine as a local
  variable instead.
  - note that in the future we will want to make Autodoc run
    simultaneously using the job system, but for now the principle of
    YAGNI dictates that we don't have an init()/deinit() API and instead
    simply call the function that does the things.
* Autodoc: only do it when there are no compile errors

ci/x86_64-linux-release.sh

@@ -40,6 +40,7 @@ cmake .. \
   -DZIG_TARGET_TRIPLE="$TARGET" \
   -DZIG_TARGET_MCPU="$MCPU" \
   -DZIG_STATIC=ON \
+  -DZIG_NO_LIB=ON \
   -GNinja
 
 # Now cmake will use zig as the C/C++ compiler. We reset the environment variables
@@ -49,6 +50,7 @@ unset CXX
 
 ninja install
 
+# TODO: move this to a build.zig step (check-fmt)
 echo "Looking for non-conforming code formatting..."
 stage3-release/bin/zig fmt --check .. \
   --exclude ../test/cases/ \
@@ -56,7 +58,9 @@ stage3-release/bin/zig fmt --check .. \
   --exclude ../build-release
 
 # simultaneously test building self-hosted without LLVM and with 32-bit arm
-stage3-release/bin/zig build -Dtarget=arm-linux-musleabihf
+stage3-release/bin/zig build \
+  -Dtarget=arm-linux-musleabihf \
+  -Dno-lib
 
 stage3-release/bin/zig build test docs \
   --maxrss 21000000000 \
@@ -68,10 +72,8 @@ stage3-release/bin/zig build test docs \
   --zig-lib-dir "$(pwd)/../lib"
 
 # Look for HTML errors.
-tidy --drop-empty-elements no -qe "stage3-release/doc/langref.html"
-
-# Produce the experimental std lib documentation.
-stage3-release/bin/zig test ../lib/std/std.zig -femit-docs -fno-emit-bin --zig-lib-dir ../lib
+# TODO: move this to a build.zig flag (-Denable-tidy)
+tidy --drop-empty-elements no -qe "zig-out/doc/langref.html"
 
 # Ensure that stage3 and stage4 are byte-for-byte identical.
 stage3-release/bin/zig build \
@@ -107,6 +109,7 @@ cmake .. \
   -DZIG_TARGET_TRIPLE="$TARGET" \
   -DZIG_TARGET_MCPU="$MCPU" \
   -DZIG_STATIC=ON \
+  -DZIG_NO_LIB=ON \
   -GNinja
 
 unset CC
@@ -118,6 +121,7 @@ stage3/bin/zig test ../test/behavior.zig -I../test
 stage3/bin/zig build -p stage4 \
   -Dstatic-llvm \
   -Dtarget=native-native-musl \
+  -Dno-lib \
   --search-prefix "$PREFIX" \
   --zig-lib-dir "$(pwd)/../lib"
 stage4/bin/zig test ../test/behavior.zig -I../test