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>

1 files changed, 12 insertions, 0 deletions

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;
+}