[REG-1483] Reorganize sidebar

[abeizer]

See plan for new sidebar layout in Notion

Changes:

• Remove "Player" section of Docs site
• Rename "Studio" section to "Developer Reference"
• Re-title sections + reorganize them per proposed notion layout
• Rename files to match their section titles
• Rename images for descriptiveness + kebab-case
• Consolidate RGFindUtils content into existing "Utilities For C# Bots" page which only contained RGGizmos
• Rename instances of "PlayTest Bot" to "Adaptive Bot"
• Rename instances of "Validation Bot" to "Scenario Bot"
• Correct all links broken by reorg
  • Note that change-log has one or two links that no longer resolve to a valid page (removed player content). This is intentional.
• Remove Studio FAQ page
• Replace sleekplan feedback links with info@regression.gg and discord link
• Add best practices to README
• Add placeholder for "Importing the RG Unity Bots Package" page

Out of scope:

• Consolidating "Creating Bots" docs into a single section whose contents change depending on a language toggle
• Changes/updates to page contents (explanations of concepts, fixing outdated information, adding APIs, etc.)

Screenshots:

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