[top_darjeeling] Doc server fails to start

Nobody has claimed this yet.

Is there a way to start the server even if some files are incomplete/invalid?

It would be really nice to be able to access the existing documentation even if some non-critical files are missing. It seems the issue here is about the test plan, which should not prevent from accessing the register definition files for example.
Thanks

```
git log -n 1
commit fcaf4d9c8862b8cbd88803bc4f812b25212bedac (HEAD -> integrated_dev, origin/integrated_dev)
Author: Adrian Lees
Date: Thu Dec 7 13:57:37 2023 +0000

./bazelisk.sh clean
WARNING: Blaze clean does not support starlark options. Ignoring options: [--@rules_rust//rust/toolchain/channel=nightly, --@rules_rust//:clippy_flags=-Aclippy::bool_assert_comparison,-Aclippy::uninlined_format_args,-Dwarnings]
INFO: Starting clean (this may take a while). Consider using --async if the clean takes more than several minutes.

./util/site/build-docs.sh serve
Build Directory : /home/eblot/Sources/Git/Rivos/opentitan/build-site
Building doxygen...
warning: Tag 'HTML_TIMESTAMP' at line 112 of file '/home/eblot/Sources/Git/Rivos/opentitan/util/doxygen/Doxyfile' has become obsolete.
To avoid this warning please remove this line from your configuration file or upgrade it using "doxygen -u"
Doxygen build complete.
INFO: Analyzed target @crate_index//:mdbook__mdbook (293 packages loaded, 7930 targets configured).
INFO: Found 1 target...
Aspect @rules_rust//rust/private:clippy.bzl%rust_clippy_aspect of @crate_index//:mdbook__mdbook up-to-date (nothing to build)
INFO: Elapsed time: 25.552s, Critical Path: 24.39s
INFO: 446 processes: 169 internal, 277 linux-sandbox.
INFO: Build completed successfully, 446 total actions
INFO: Build completed successfully, 446 total actions
2023-12-08 14:53:53 [INFO] (mdbook::book): Book building has started
Error: No testpoints or covergroups found in /home/eblot/Sources/Git/Rivos/opentitan/sw/device/silicon_creator/manuf/data/manuf_testplan.hjson
2023-12-08 14:54:04 [ERROR] (mdbook::utils): Error: The "testplan" preprocessor exited unsuccessfully with exit status: 1 status
```

Start with util/site/build-docs.sh and reproduce the failure using its serve command. Inspect how the documentation build invokes the testplan preprocessor for sw/device/silicon_creator/manuf/data/manuf_testplan.hjson, then verify that an invalid or incomplete non-critical file no longer prevents the remaining documentation from being served.

Written by the indexing model from the issue text.