[skip ci] fix the index file in docs
<ul>
<!-- links to other docs -->
<li>
- <a href="https://cryptape.github.io/">Nervos</a>
+ <a href="https://cryptape.github.io/" target="_about">Nervos</a>
<ul>
<li>
- <a href="https://cryptape.github.io/Nervos-AppChain-Docs/">AppChain</a>
+ <a href="https://cryptape.github.io/Nervos-AppChain-Docs/" target="_about">AppChain</a>
</li>
<li>
<a href="https://cryptape.github.io/cita/">CITA</a>
</li>
<li>
- <a href="https://cryptape.github.io/Neuron-Android/#/">Neuron</a>
+ <a href="https://cryptape.github.io/Neuron-Android/#/" target="_about">Neuron</a>
</li>
<li>
- <a href="https://cryptape.github.io/Microscope/">Microscope</a>
+ <a href="https://cryptape.github.io/Microscope/" target="_about">Microscope</a>
</li>
</ul>
</li>
