pg_basebackup: add page

[harryfrzz]

• The page(s) are in the correct platform directories: common, linux, osx, windows, sunos, android, etc.
• The page description(s) have links to documentation or a homepage.
• The page(s) follow the content guidelines.
• The page(s) follow the style guide.
• The PR contains at most 5 new pages.
• The PR title conforms to the recommended templates.
• Version of the command being documented (if known):

[dmmqz]

Please add long options for the commands (e.g. -h -> {{[-h|--host]}}).

[dmmqz]

LGTM after this.

[harryfrzz]

@Managor Please take a look at the changes made