Update docs on age in metadata

[allyhawkins]

Based on https://github.com/AlexsLemonade/ScPCA-admin/issues/922, we will need to make two changes to the existing tables that mention metadata:

• The age_at_diagnosis column should be renamed age
• Add new age_timing column

Here are the spots that should be updated:

• Download files page
• SCE file contents - sample metadata
• merged objects

[allyhawkins]

Closed by #331