Add timeout fine-tunning advanced docs section

[mwoss]

Reference issue #434

I've added a small intro to HTTPX timeouts, sections with examples (plain request, client, timeout config object) and a paragraph about disabling timeouts.

btw, I apologize for any grammar mistakes, English is my second language.

[florimondmanca]

Thanks for this and congrats on the write up! I left a couple of phrasing suggestions. Excited about how helpful this will be for users. :-)

[mwoss]

Done. I rephrased most of the dosc, using your suggestions (more like copying it) - I really like your writing style. :)

[florimondmanca]

Looking good to me -- thanks again for putting this together!