Docs polishing

[LukeMathWalker]

Some changes to our docs:

• Removed linxal mentions, given that it doesn't seem to be maintained/actively developed (the latest commit is 2 years old);
• Added a proper ecosystem section in the docs landing page and in the introduction for NumPy users, to make it easier to find auxiliary crates (see Adding Norms #694)
• Minor updates to the introduction for NumPy users, with new constructors and other API changes we have made since the last time we touched it.

[bluss]

Thanks!