Improve reverse-proxy document and fix nginx config bug

[wxiaoguang]

Close #23711, thanks to @ghnp5 !
Close #24612, thanks to @DanielGibson !

Major changes:

• the default value of nginx's client_max_body_size is too small, so put a 512M here
• move Resolving Error: 413 Request Entity Too Large to a sub-section of Nginx section
• make nginx use unescaped the URI and keep "%2F" as is when using sub-path
• add details for General sub-path configuration

[silverwind]

Am a bit sceptical regarding thowse rewrites, but I don't know another solution off my head at least because nginx slash handling is generally weird.

[DanielGibson]

I think the documentation on client_max_body_size could be more explicit about it not only affecting file uploads in the webui, but also via git when using LFS (=> keep the If you use Git LFS, this will also limit the size of the largest file you will be able to push. sentence from the prior version).
For lazy people who don't want to look at the nginx doc you could also mention the special case of 0 meaning no limit ;)

[wxiaoguang]

I think the documentation on client_max_body_size could be more explicit about it not only affecting file uploads in the webui,

8298848

For lazy people who don't want to look at the nginx doc you could also mention the special case of 0 meaning no limit ;)

"nolimit" would fill up user's disk if the user runs nginx with tmp dir in a small file system.