what is silence in audio .wav file and how to compute decibels?
Q1: Sample values in .wav file represent the waveform amplitude?

Yes.

Q2: What is silence?

That depends on how you define it. The signal that you generated actually has a lot of energy but it's all at 0 Hz. Your DAC cannot reproduce 0 Hz, your speaker cannot reproduce 0 Hz, air cannot transmit 0 Hz, and your ear can't hear 0 Hz. There is lots of energy but you can't get it to you ear or perceive it. If you want a loud signal, create a sine wave with a frequency 1000 Hz and an amplitude of 10000. That will be loud.

Q3: How to compute decibels from the .wav amplitude values?

That depends on what type of decibels and for what purpose. You can always calculate dBFS (dB re digital Full Scale) as
$$20 \cdot log(x/x_{max})$$
where $x$ is your sample and $x_{max}$ the clipping point of your AD or DA converter, i.e. 32768 for signed 16-bit integers.
Sound pressure levels, dBSPL, are much more complicated and require calibration and in many cases also a frequency weighting function.