doc/guide: update content and sample code (#3408)

- Add `Makefile` for example codes. (cherry-pick from old uvbook repo)
- Add a new example "Default loop" to "Basics of libuv"/"Default loop"
- Document review and update: `Introduction`, `Basics of libuv`, `Filesystem`
    + Update the referenced libuv code snippet
    + Link update: http->https
    **Content Updates**:
    - `filesystem.rst`#L291-L297: Add note for `uv_fs_event_start`
    - `filesystem.rst`#L334: Add description of the callback function parameter `status`

The following examples have been tested manually in WSL2 (Ubuntu 20.04) with libuv 1.42.0:
- helloworld
- default-loop
- idle-basic
- uvcat
- uvtee
- onchange (test on macOS)

Co-authored-by: Nikhil Marathe <nsm.nikhil@gmail.com>

3 files changed, 97 insertions, 0 deletions

diff --git a/docs/code/.gitignore b/docs/code/.gitignore
new file mode 100644
index 00000000..c46ecde4
--- /dev/null
+++ b/docs/code/.gitignore
@@ -0,0 +1,3 @@
+*/*
+!*.c
+!*.h
diff --git a/docs/code/Makefile b/docs/code/Makefile
new file mode 100644
index 00000000..0526e8c2
--- /dev/null
+++ b/docs/code/Makefile
@@ -0,0 +1,82 @@
+examples=\
+	helloworld\
+	default-loop\
+	idle-basic\
+	uvcat\
+	uvtee\
+	onchange\
+	thread-create\
+	queue-work\
+	progress\
+	tcp-echo-server\
+	dns\
+	udp-dhcp\
+	idle-compute\
+	ref-timer\
+	spawn\
+	detach\
+	proc-streams\
+	cgi\
+	pipe-echo-server\
+	multi-echo-server\
+	tty\
+	tty-gravity\
+	interfaces\
+	locks \
+	signal \
+	uvstop \
+	queue-cancel
+
+UV_PATH=$(shell pwd)/../..
+UV_LIB=$(UV_PATH)/.libs/libuv.a
+CFLAGS=-g -Wall -I$(UV_PATH)/include
+LIBS=
+
+uname_S=$(shell uname -s)
+
+ifeq (Darwin, $(uname_S))
+CFLAGS+=-framework CoreServices
+SHARED_LIB_FLAGS=-bundle -undefined dynamic_lookup -o plugin/libhello.dylib
+endif
+
+ifeq (Linux, $(uname_S))
+LIBS=-lrt -ldl -lm -pthread -lcurl
+SHARED_LIB_FLAGS=-shared -Wl,-soname,libhello.so -o plugin/libhello.so
+PLUGIN_EXE_FLAGS=-Wl,-export-dynamic
+endif
+
+
+all: $(examples) plugin/plugin proc-streams/test cgi/tick multi-echo-server/worker uvwget/uvwget
+
+$(examples): % : %/main.c
+	gcc $(CFLAGS) -o $@/$@  $< $(UV_LIB) $(LIBS)
+
+plugin: plugin/plugin
+plugin/plugin: plugin/*.c
+	gcc $(CFLAGS) $(PLUGIN_EXE_FLAGS) -o plugin/plugin plugin/main.c $(UV_LIB) $(LIBS)
+	gcc -g -Wall -c -fPIC -o plugin/hello.o plugin/hello.c
+	gcc $(SHARED_LIB_FLAGS) plugin/hello.o
+
+proc-streams/test: proc-streams/test.c
+	gcc -g -Wall -o proc-streams/test proc-streams/test.c
+
+cgi/tick: cgi/tick.c
+	gcc -g -Wall -o cgi/tick cgi/tick.c
+
+multi-echo-server/worker: multi-echo-server/worker.c
+	gcc $(CFLAGS) -o multi-echo-server/worker multi-echo-server/worker.c $(UV_LIB) $(LIBS)
+
+uvwget: uvwget/uvwget
+uvwget/uvwget: uvwget/main.c
+	gcc $(CFLAGS) `curl-config --cflags --libs` -o uvwget/uvwget uvwget/main.c $(UV_LIB) $(LIBS)
+
+clean:
+	for dir in $(examples); do cd $$dir; rm -f $$dir; rm -rf $$dir.dSYM; cd ..; done
+	rm -rf plugin/*.o plugin/libhello.*
+	rm -rf plugin/plugin plugin/plugin.dSYM
+	rm -rf proc-streams/test proc-streams/test.dSYM
+	rm -rf cgi/tick cgi/tick.dSYM
+	rm -rf multi-echo-server/worker multi-echo-server/worker.dSYM
+	rm -rf uvwget/uvwget uvwget/uvwget.dSYM
+
+.PHONY: clean all $(examples) plugin uvwget
diff --git a/docs/code/default-loop/main.c b/docs/code/default-loop/main.c
new file mode 100644
index 00000000..e00a4d2b
--- /dev/null
+++ b/docs/code/default-loop/main.c
@@ -0,0 +1,12 @@
+#include <stdio.h>
+#include <uv.h>
+
+int main() {
+    uv_loop_t *loop = uv_default_loop();
+
+    printf("Default loop.\n");
+    uv_run(loop, UV_RUN_DEFAULT);
+
+    uv_loop_close(loop);
+    return 0;
+}