Subnet 9000 Replacement

[ashucoder9]

• [x] Replace Subnet with Avalanche L1 in content without replacing commands
• [x] Modify files and folder names
• [x] Reflect file name changes in the link structure across modified pages
• [x] Make sure content like Subnet-EVM is untouched while replacement
• [x] Make sure the context of "Subnet = Set of Validators" is maintained throughout the docs
• [x] Modify redirects to reflect the latest changes

PS: manually update the words wherever a better context is required while making the changes.

also, the remote docs are not updated yet (specifically cross-chain folder atm).

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Name	Status	Preview	Comments	Updated (UTC)
avalanche-docs	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Sep 25, 2024 8:24am

[ashucoder9]

@owenwahlgren can you approve the PR and merge?