Consolidate security and privacy documents

[cakrit]

Reorganize info into two main documents:

• An overall security and privacy design, under docs/netdata-security.md
• A "secure your nodes" category page, under docs/category-overview-pages/secure-nodes.md, with all the options for securing netdata agents summarized (additional pages with more details are available under that category in learn).

This PR also removes:

• The recently created doc on HIPAA, SOC2 etc. and
• The older privacy document that just listed the metadata we have and how to delete your account