Manual install instruction

The install instruction in the manual still used an old `configure` setup. I copied and adjusted the instructions from `README.md`. It would be nice if a native speaker could look over it. Thanks
The [`test/DocTests.py`](https://github.com/ledger/ledger/blob/master/test/DocTests.py) script was introduced with ledger/ledger@fbbb379fe08b051b40c071041108a2526533f417 and is being run by CMake since then. Maybe your source tree isn't up-to-date with the ledger master branch?

Another way of running the tests is `python test/DocTests.py -l ${PATH_TO_LEDGER_BINARY} -f doc/ledger3.texi`, which is what CMake is doing internally.