[docs] Migrate Files API to /docs/api/files

[andrewnicols]

The URL of the page on https://docs.moodle.org/dev/

https://docs.moodle.org/dev/File_API

Is this documentation specific to a Moodle version?

Yes

What location would you suggest in the new docs?

/docs/api/files/index.md

We need to migrate:

• [x] File_API #59
• [x] FIle_API_internals #59
• [ ] Using_the_File_API_in_Moodle_forms

These should all be a part of the /docs/api/files subtree

[ziegenberg]

The link to the migrated Moodle Developer Resources on this page https://docs.moodle.org/dev/File_API in MediaWiki is broken. It's currently pointing to https://moodle.github.io/devdocs/docs/subsystems/apis/files but the correct link would be https://moodle.github.io/devdocs/docs/apis/subsystems/files. The page in MediaWiki is protected, which is why I wasn't able to fix it myself.