added citation page to docs

[agladstein]

Description

Added recommended citations for tskit to the docs. I modeled it after the msprime docs. These were the citations mentioned in a slack conversation.

Let me know if you want any changes to the language or any different citations.

(No associated issue)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.72%. Comparing base (feeecb5) to head (5553675).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3143      +/-   ##
==========================================
- Coverage   89.94%   89.72%   -0.23%     
==========================================
  Files          29       29              
  Lines       32651    32225     -426     
  Branches     5854     5890      +36     
==========================================
- Hits        29368    28913     -455     
- Misses       1869     1882      +13     
- Partials     1414     1430      +16     

Flag	Coverage Δ
c-tests	86.66% <ø> (ø)
lwt-tests	80.38% <ø> (-0.40%)	⬇️
python-c-tests	88.24% <ø> (-0.99%)	⬇️
python-tests	98.83% <ø> (ø)

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see 3 files with indirect coverage changes

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[benjeffery]

LGTM! Thanks!