Add documentation for AddPathPlugin and BaseUriPlugin

[Nyholm]

... and also for the AddHostPlugin

I believe we should have one page for all these 3 plugins because they are very related.

Ping @soullivaneuh

Related to php-http/client-common#48

[soullivaneuh]

I believe we should have one page for all these 3 plugins because they are very related.

What name for the page? Uri manipulation?

[Nyholm]

BaseUriPlugin?

"The BaseUriPlugin consist of two other plugins..."

[dbu]

oh, AddHostPlugin was not documented at all. maybe "Request URI Manipulations"?

[soullivaneuh]

oh, AddHostPlugin was not documented at all

Yeah, I found the documentation by listing the Plugins source directory.

maybe "Request URI Manipulations"?

I like that.