Add link to optional dependencies installation guide

[lbianchi-lbl]

Resolves #1274

Changes proposed in this PR:

• Add link from "Getting started" installation tutorial to optional dependencies installation guide

Legal Acknowledgement

By contributing to this software project, I agree to the following terms and conditions for my contribution:

1. I agree my contributions are submitted under the license terms described in the LICENSE.txt file at the top level of this directory.
2. I represent I am authorized to make the contributions and grant the license. If my employer has rights to intellectual property that includes these contributions, I represent that I have received permission to make contributions and grant the required license on behalf of that employer.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (eb71ca3) 77.25% compared to head (17ec564) 77.24%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1279      +/-   ##
==========================================
- Coverage   77.25%   77.24%   -0.01%     
==========================================
  Files         387      387              
  Lines       63127    63127              
  Branches    11613    11613              
==========================================
- Hits        48768    48765       -3     
- Misses      11837    11841       +4     
+ Partials     2522     2521       -1     

see 1 file with indirect coverage changes

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[ksbeattie]

LGTM