Replaced old codex link with new resources

[Dhruval-678]

Fixes : #597

[swissspidy]

There are tons of links to the old codex: https://github.com/search?q=repo%3Awp-cli%2Fhandbook+codex.wordpress.org&type=code

Wanna help change all of them?

[Dhruval-678]

@swissspidy Sure will do

[Dhruval-678]

@swissspidy
As suggested replace all old links.

[mrsdizzie]

I think many of these need to actually change in the command DocBlock where the pages are generated from, not in this project. ie:

https://github.com/wp-cli/cache-command/blob/408bde47b7c19d5701d9cb3c3b1ec90fb70295cd/src/Cache_Command.php#L13-L14

Internal API docs and command pages are generated dynamically from the WP-CLI codebase using the wp handbook series of commands.