Add 'usage' section with explanation of 'dump' command

[issmirnov]

As per go-gitea/gitea#448, sending a PR that adds a new 'Usage' section to the docs and documents the behavior of the dump command.

Please let me know what you think of the phrasing and structure.

Looking forward, it would be nice to have explanations of all the commands added to these docs. Right now this information is only present in the CLI help:

     web      Start Gitea web server
     serv     This command should only be called by SSH shell
     update   This command should only be called by Git hook
     dump     Dump Gitea files and database
     cert     Generate self-signed certificate
     admin    Preform admin operations on command line
     help, h  Shows a list of commands or help for one command

IMO, web cert and admin would be the most useful.

[lunny]

LGTM

[Bwko]

LGTM

[lunny]

@issmirnov could you send this PR also to release/v1.0?

[bkcsoft]

This is wrong, it's not a "mysql dump" it's a plain "SQL dump" for any database that you happen to use.

[lunny]

And dump has a new option -d=mysql, that means you can backup your sqlite as mysql SQL syntax.