Docs: Document built-in functions

[kmr-srbh]

The following functions were documented:

• abs(x)
• bin(n)
• complex(x=0, y=0)
• divmod(x, y)
• exp(x)
• hex(n)
• len(s)
• max(x, y)
• min(x, y)
• mod(x, y)
• pow(x, y)
• round(x)
• sum(arr)
• oct(n)

[kmr-srbh]

I think that this is good start. Thanks!

@certik could you please guide me on https://lfortran.zulipchat.com/#narrow/stream/311866-LPython/topic/Documentation/near/431329196 also? I wish to document the ASR Utility functions as they are very useful.