Link non-conda install methods to respective docs

[eharkins]

Description of proposed changes

This replaces the specific instructions of how to install augur and auspice using pip and npm respectively with links to their own documentations, so that these instructions don't become out of sync with the latest install practices for either tool.

This is one of multiple such PRs I will be submitting as we try to deduplicate things between the nextstrain.org docs and other docs, while aiming not to have too many links buried in text on the nextstrain.org docs such that they are not found when folks are looking for them.

One strategy I've discussed with @jameshadfield to accomplish that aim is to (where possible) instead of replacing some chunk of text in a given document with an external link, try to break up the information in that document into seperate documents so that sections or even entire documents/pages on the nextstrain.org docs can be replaced with an external link. This allows us to prominently display that link in the sidebar hierarchy of topics and avoid burying it the document text somewhere.

The difference might look something like this, going from having links in the document:

to removing those sections from the document and replacing them with links in the sidebar as subheadings of Local Installation:

This also allows for a more straightforward swap if we want to try to inject the content from that external (e.g. auspice) document into the nextstrain.org document to improve the docs user experience.

Testing

Just tested locally with gatsby.

Thank you for contributing to Nextstrain!

[jameshadfield]

Thanks @eharkins - this will really help with things getting out of sync. Having one canonical install instructions & the rest as external links is a nice balance. The expand/collapse functionality will come in useful in the future, too.