Create a list of relevant feeds and assets for Secure

As part of our technical onboarding, it would be useful to provide a list of sources to newcomers: books to read, mailing lists, blogs, Twitter accounts to subscribe to, Computer Science subjects to look at, etc.

We should have a page in our handbook section (https://about.gitlab.com/handbook/engineering/ops-backend/secure/) with that information. This page would also be useful for the team to share useful content and make sure we are raising the level of general knowledge. Thanks @vzagorodny for the suggestion!