Linux platform example app includes SHELL unconditionally. (#7205)

* Linux platform example app includes SHELL unconditionally.

Force defines and linkages as such, otherwise compilation with GN
without 'use shell' does not work.

TODO as followup: if shell is really optional, build it as such in linux
as well.

* remove the enable shell arg alltogether - it does not work because includes are already there

* Revert "Fix relative path for the markdown link (#7202)"

This reverts commit 7a172f3ee35cb396c5bad9491ec18c1e30a92322.
Apparently link checker is not fully working and this broke doxygen.
diff --git a/docs/README.md b/docs/README.md
index c7a9d2a..ef6e622 100644
--- a/docs/README.md
+++ b/docs/README.md
@@ -6,7 +6,7 @@
     [the build guide](./BUILDING.md)
 -   Documentation about running [cirque](https://github.com/openweave/cirque)
     tests can be found in
-    [the cirque test guide](../src/test_driver/linux-cirque/README.md)
+    [the cirque test guide](src/test_driver/linux-cirque/README.md)
 -   Documentation about standard build & development flows using
     [Visual Studio Code](https://code.visualstudio.com/) can be found in
     [the development guide](./VSCODE_DEVELOPMENT.md)