Fix broken links about linux sandboxing

/docs/linux/sandboxing.md file is changed to /src/sandbox/linux/README.md at this below commit.
https://chromium-review.googlesource.com/c/chromium/src/+/5597924

And fix additional broken link in docs/design/sandbox.md
This also is related to sandboxing.md

Bug: None
Change-Id: Ib75aba447659fd7cdc8e3552c94e38205d13f1f8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6507951
Commit-Queue: Euisang Lim <[email protected]>
Reviewed-by: Euisang Lim <[email protected]>
Reviewed-by: Elly FJ <[email protected]>
Cr-Commit-Position: refs/heads/main@{#1455951}
diff --git a/docs/README.md b/docs/README.md
index 9daf487c..a6c31809 100644
--- a/docs/README.md
+++ b/docs/README.md
@@ -259,7 +259,7 @@
     related tools.
 *   [Chroot Notes](linux/using_a_chroot.md) - Setting up a chroot to work around
     libfreetype differences in some versions of Linux.
-*   [Linux Sandboxing](linux/sandboxing.md) - The Linux multi-process model to
+*   [Linux Sandboxing](../sandbox/linux/README.md) - The Linux multi-process model to
     isolate browser components with different privileges.
 *   [Zygote Process](linux/zygote.md) - How the Linux Zygote process, used to
     spawn new processes, works.