doxygen: support running from the root directory

Previously, you had to enter the documentation directory
before executing the run.sh script, which was not very
convenient during development, especially when you needed
to execute other commands in the source code root directory
at the same time, such as git commands.

Now you can directly run script in the source code root
directory as below:

```shell
$ cd $RTT
$ ./documentation/run.sh
```

No need to switch the working path any more after entering
the source code root directory.

Signed-off-by: Chen Wang <[email protected]>

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