Clean up the secrets page

[ppiegaze]

The secrets page (content/user-guide/task-configuration/secrets.md) needs to be cleaned up an rewritten. Specifically, we need to make sure that the file-based secrets mechanism is correctly described. Currently a lot of this is commented out. Please un-comment these parts, clean uyp and if possible consult the repo https://github.com/flyteorg/flyte-sdk for more details on how secrets work in flyte