Doxygen: Add searchbar

[t-b]

If you think this is a good change then please also update https://tango-controls.github.io/cppTango-docs/.

[bourtemb]

Sounds like a good idea...
I'm wondering why we are not using the default value here... I guess probably because the default value used to be NO in the past.
Did you get the opportunity to test it with cppTango?
The documentation about this parameter is mentioning that the search can be very slow with big projects... I don't know whether cppTango falls into that category?
I guess it also depends on the hardware capabilities on the client side... Nowadays, people have computers with much more memory than in the past.

[bourtemb]

I just tried it on a computer with 4GB RAM and it is very responsive. I think this is indeed a very useful feature and an improvement.

[t-b]

@bourtemb It is very responsive here. But I also do have a powerful machine.