Improve explanations of profiles

[jfy133]

Describe the bug
Received some comments that for someone digging deeper into profiles and how they work (ordering, where defaults are stored etc.) is hard to find/understand

• Give disclaimer on 'running' docs to see 'installation' for more details
• Explain where default EAGER2 profiles are stored
• Explain order precendence
• Adding your own system section on 'installatoin' should point to nf-core/configs