Implement new FanGraphs Leaderboard API

[ss77995ss]

Description

• FanGraphs update their leaderboard page in 2023. Therefore the old data scraping does not work. Currently, we use their legacy url https://www.fangraphs.com/leaders-legacy.aspx now. But the legacy one might not update after 2023.
  • ref: https://blogs.fangraphs.com/weve-updated-our-major-league-leaderboards-interface/
  • related issue: https://github.com/jldbc/pybaseball/issues/376
• To use latest leaderboard, apply fangraph's leaderboard api to get the data
  • url: link
  • commit: https://github.com/jldbc/pybaseball/commit/0f39311b63df54c50eac0d1e2eed16cdd9b73c56
• Add a legacy flag to remain obtaining data from legacy leaderboard. Also add a notification in document
  • commit: https://github.com/jldbc/pybaseball/commit/7205275fdf4a53abcf70df186a4930625c3014e5, https://github.com/jldbc/pybaseball/commit/ec9e2bc4c5cc84749f30904ec33df0b042817084

Need Discussions

• Currently all the params seem to work but I am not check completely. Might need more testing.
• Test will not pass since the response data is different between legacy and latest leaderboard. Need some help for the testing data
• Look like issue: https://github.com/jldbc/pybaseball/issues/396 can also be fixed if we use fangraphs api link

TODO

• [ ] Fix testing
• [ ] Delete demo commit