$$
\begin{align}
\int_0^1\log(1+x)\frac{\mathrm{d}x}{1+x^2}
&=\int_0^1\log\left(\frac2{1+x}\right)\frac{\mathrm{d}x}{1+x^2}\\
&=\frac\pi8\log(2)\tag{1}
\end{align}
$$
$$
f(a)=\int_0^1x\log(a+x)\frac{\mathrm{d}x}{1+x^2}\tag{2}
$$
$$
\begin{align}
f(0)
&=\int_0^1x\log(x)\frac{\mathrm{d}x}{1+x^2}\\
&=\frac12\int_0^1\log(x)\mathrm{d}\log(1+x^2)\\
&=-\frac12\int_0^1\frac{\log(1+x^2)}{x}\mathrm{d}x\\
&=-\frac{\pi^2}{48}\tag{3}
\end{align}
$$
$$
\begin{align}
f'(a)
&=\int_0^1\left[\frac{ax+1}{1+x^2}-\frac{a}{a+x}\right]\frac{\mathrm{d}x}{1+a^2}\\