Added installation instructions to developer documentation

[jtboing]

Description of proposed changes

This pull request adds complete installation instructions in the developer documentation (dev docs) and exposes dev docs by adding links in sections related contributing to the dev docs and adding the dev docs to the main augur docs.

Related issue(s)

Related to #664

Testing

Confirmation that documentation builds

[codecov[bot]]

Codecov Report

Merging #686 (1582870) into master (867c5e3) will increase coverage by 0.92%. The diff coverage is n/a.

:exclamation: Current head 1582870 differs from pull request most recent head 804c025. Consider uploading reports for the commit 804c025 to get more accurate results

@@            Coverage Diff             @@
##           master     #686      +/-   ##
==========================================
+ Coverage   29.34%   30.27%   +0.92%     
==========================================
  Files          39       40       +1     
  Lines        5455     5563     +108     
  Branches     1335     1354      +19     
==========================================
+ Hits         1601     1684      +83     
- Misses       3796     3819      +23     
- Partials       58       60       +2     

Impacted Files	Coverage Δ
augur/ancestral.py	10.34% <0.00%> (-0.55%)	:arrow_down:
augur/__init__.py	31.25% <0.00%> (ø)
augur/index.py	82.14% <0.00%> (ø)
augur/align.py	91.28% <0.00%> (+0.04%)	:arrow_up:
augur/util_support/shell_command_runner.py	91.66% <0.00%> (+0.17%)	:arrow_up:
augur/filter.py	47.04% <0.00%> (+1.98%)	:arrow_up:

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[jtboing]

Hello, everyone. I've adjusted this pull request according to the requested changes and I have also tried the installation steps successfully. I apologize if I am very late. Please comment on the code if any further changes are necessary. Thank you.