docs: rebrand btcd references to utreexod across documentation

[TechLateef]

Fixes #416

The docs/ directory contained numerous stale references to the original btcd project, which was confusing for new users trying to understand and run utreexod.

This PR performs a comprehensive cleanup of the documentation by:

• Rebranding btcd to utreexod
• Rebranding the btcctl CLI tool to utreexoctl
• Updating GitHub and GoDoc URLs from btcsuite to utreexo
• Updating configuration paths (~/.btcd -> ~/.utreexod, btcd.conf -> utreexod.conf)

Note: I intentionally left the blog post links intact to preserve historical references, and I left the btcwallet references untouched as those will be handled in a separate PR.

[kcalvinalvin]

There would be a conflict with #427 when that lands so I'll review this after that lands

[kcalvinalvin]

I'll take another look after you rebase

[TechLateef]

I'll take another look after you rebase

Done, I’ve rebased the branch. You can take another look Thank you.

[TechLateef]

Hello! @kcalvinalvin I've updated the PR to address the issues:

• Reverted the Windows addblock path to %PROGRAMFILES%\Btcd Suite\Btcd\addblock in docs/configuration.md.
• Reverted the debuglevel subsystem to BTCD in docs/json_rpc_api.md to match log.go.
• Reverted utreexodjsonrpcapi to btcdjsonrpcapi in docs/json_rpc_api.md to match the actual wire response in rpcserver.go.