Add Clio ledger_index API method [draft]

[mDuo13]

Documents XRPLF/clio#1503, which adds a new ledger_index API method. Expected in the (upcoming) Clio 2.3.0 release.

Also removes the deprecated fields from the ledger API example.

• Move Clio Methods index page to appropriate place and update it accordingly, leaving a redirect for the old URL.
• Add ledger_index method definition page
• Add ledger_index to WebSocket tool
• Update WS tool to remove deprecated fields from ledger method (see also, ledger (Clio) - update deprecated fields note for 2.2.2 #2701)
• Wait for Clio 2.3.0 release
• Add example response data
• Clarify restrictions on date
• Clarify closed field from response

[kennyzlei]

@cindyyan317 to help review prior to Clio 2.3.0 release