Update getting-started.rst to fix installation link

[ocundale]

Fix broken link in docs, replace /html/ with /latest/

[earthgecko]

Thanks @ocundale 👍 although to align it with the general linking convention now used this should be:

Getting started
===============

See the `Installation <installation.html>`__ page **after** reviewing the below.

This is now the method used rather than defining the link, in these cases when linking within the docs. This ensures that the link is relevant to the version, rather than hardcoding latest.

Further this works within the Skyline webapp's own docs which it hosts, this was the original reason for the /html/ link, using /latest/ would break the self hosted docs on Skyline.