Update conda install instructions & listed dependencies

[saitcakmak]

Motivation

As noted in #1206, current conda installation instructions do not work for latest versions since the listed channels cannot resolve the pyro-ppl dependency. This updates the instructions to include the conda-forge channel as a fix.

Note: Technically conda-forge has botorch and gpytorch as well, so we could get away without including -c pytorch and -c gpytorch. However, conda-forge versions of these packages may not always be up-to-date, so keeping these channels makes sure we can always install the latest versions.

Test Plan

./scripts/build_docs.sh

[facebook-github-bot]

@saitcakmak has imported this pull request. If you are a Meta employee, you can view this diff on Phabricator.

[Balandat]

lgtm, thanks!

[codecov]

Codecov Report

Merging #1207 (9468bb8) into main (0080c89) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main    #1207   +/-   ##
=======================================
  Coverage   99.99%   99.99%           
=======================================
  Files         115      115           
  Lines       10176    10176           
=======================================
  Hits        10175    10175           
  Misses          1        1           

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