Mention only the core packages here

[tmigot]

I think we should talk only about the core packages here, and move the extra information to the website. Otherwise, the first contact is too overwhelming.

Instead of focusing on the package, we could focus on the target audience:

• "I want to solve a linear system" -> Go here
• "I want to solve an optimization problem" -> Go here
• "I want to build an optimization method" -> Go here

Originally posted by @abelsiqueira in #1 (comment)