Update Learn section highlights

According to @PeterTurcan assessment in https://github.com/boostorg/website-v2-docs/issues/244:

**User Guide Learn page:**

1. Remove these two links completely

Library Names & Organization
Header & Compiled Binaries

2. Add the following text and link:
Text: Introduction to the User Community
Link: https://www.boost.io/doc/user-guide/user-community-introduction.html

**Contributor Guide Learn page:**

1. Design Guide link is broken - change it to:
https://www.boost.io/doc/contributor-guide/design-guide/design-best-practices.html

2. After "Getting Involved" - add a link to the Contributors FAQ:
Text: Contributors FAQ
Link: https://www.boost.io/doc/contributor-guide/contributors-faq.html

3. Add a link to the Super-Project
Text: The Super-Project
Link: https://www.boost.io/doc/contributor-guide/superproject/overview.html

4. Change the text from "Libraries" to "Library Requirements" - no other changes to this one

5. Remove the text and links for these (as they are accessed from Library Requirements)
Licenses
Portability
Code Organization

6. Add the text and link:
Text: Introduction to the Contributors Community
Link: https://www.boost.io/doc/contributor-guide/contributor-community-introduction.html

These changes should result in both sections containing 8 links into the docs, and be fairly robust going forward.