feat(cmd/gf): add command gf doc for local documentation

[hailaz]

下载pages.goframe.org到本地运行

GOPROXY=direct GOSUMDB=off go install github.com/gogf/gf/cmd/gf/v2@feat/docrun
gf doc

[gqcn]

@hailaz 这个下载全量的pages文档有些慢，得161MB。既然要去github下载pages文档，那为什么不直接去访问pages呢，单独访问pages还不需要全量下载？

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@hailaz This download of the full pages document is a bit slow, it is 161MB. Since you are going to github to download the pages document, why not directly access pages? Accessing pages alone does not require full downloading?

[hailaz]

@hailaz 这个下载全量的pages文档有些慢，得161MB。既然要去github下载pages文档，那为什么不直接去访问pages呢，单独访问pages还不需要全量下载？

强哥有所不知，并不是所有人都有稳定的梯子，要不是有goproxy，有的人代码都难下载。
现在官网在新加坡，已经拦截掉一些人了，pages访问也受墙影响，最后http的访问比pdf好。
我新加了个proxy参数，可以用代理来下载整个文档。
加这样一个功能并没有坏处吧，只有第一次下载费时间，第二次就直接启动了。

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@hailaz This download of the full pages document is a bit slow, 161MB. Since you are going to github to download the pages document, why not directly access pages? Accessing pages alone does not require full downloading?

What Brother Qiang doesn’t know is that not everyone has a stable ladder. If it weren’t for goproxy, it would be difficult for some people to download the code.
Now that the official website is in Singapore, some people have been blocked. Pages access is also affected by the wall. Finally, http access is better than pdf.
I added a new proxy parameter, which can use a proxy to download the entire document.
There is no harm in adding such a function. It only takes time to download the first time, but it starts directly the second time.

[houseme]

LGTM