Fix DocC generation

[0xTim]

Fix DocC generation. Force Linux build on SPI to get it to render the Crypto 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:

Currently the Crypto docs are empty on SPI because they're built on macOS where there are no symbols. CryptoExtras index page also doesn't work because of a name mismatch

Modifications:

Force Linux build on SPI, fix name for extras documentation

Result:

Working Docs

[0xTim]

@Lukasa another small one for you, verified locally

[Lukasa]

Nice one, thanks!