Generate index.json for static search

[nexero]

In order to do a static search without server backend, we need to implement the search routines in JS like described in RDTK/CITKat#6.
To do that, we need a file that contains all file names to search through, e.g. our directory and files structure looks like this:

$ tree
.
├── distribution
│   ├── lsp-famula-nightly.xml
│   └── lsp-famula-sanitized.xml
├── index.json
└── project
    └── boost_threadpool-0.2.5.xml

2 directories, 4 files

The content of an index.json should look like this then (must not contain index.json itself):

[
      "distribution/lsp-famula-nightly",
      "distribution/lsp-famula-sanitized",
      "project/boost_threadpool-0.2.5"
]