Improve DocC landing pages

[fpseverino]

Improve DocC landing pages by mimicking Apple's CryptoKit docs

Checklist

• I've run tests to see all new and existing tests pass
• I've followed the code style of the rest of the project
• I've read the Contribution Guidelines
• I've updated the documentation if necessary

If you've made changes to gyb files

• I've run .script/generate_boilerplate_files_with_gyb and included updated generated files in a commit of this pull request

Motivation:

Right now the DocC docs group all APIs by protocols, structures and enums.
By adding a topics section in the landing page we can group them by type ("Ciphers", "Public key cryptography", etc.) exactly in the way Apple does in CryptoKit docs.

Modifications:

• Fix DocC compilation errors in _CryptoExtras
• Add topics section in _CryptoExtras landing page
• Add topics section in Crypto landing page

Result:

APIs will be grouped by type.

[Lukasa]

Really nice @fpseverino, thanks!