Add clinical data download guide

[guanqiaofeng]

Users complain about no documentation for clinical data download

Detailed Description

Two issues:

1. This section in our doc site (https://docs.icgc-argo.org/docs/data-access/data-download#searching-for-files) NOTE: Clinical data can be downloaded by any user and does not require the score-client is misleading as it indicates that clinical data download does not require DACO approval, which is not true. Need to update this section
2. We do not have documentation for clinical data download. Adding this section will be very useful for users