doc: use BGL commands in I2P guide

[pinehill99]

Summary:

• Update doc/i2p.md to use BGL Core, BGLd, and BGL-cli in user-facing I2P instructions.
• Keep the inherited I2P protocol details and external references unchanged.

Validation:

• rg -n -e 'Bitcoin Core' -e 'bitcoind' -e 'bitcoin-cli' -e 'Bitcoin application' -e 'with Bitcoin' doc/i2p.md
• git diff --check

Bounty:

• Refs BGL PR bounty hunt ($10000 overall budget) #39