doc: align fuzzing guide with BGL Core

[pinehill99]

Summary:

• Update doc/fuzzing.md quickstarts to clone BitgesellOfficial/bitgesell and use the bitgesell checkout name.
• Align fuzzing guide headings and BGLd examples with BGL Core while keeping upstream qa-assets and OSS-Fuzz references explicit.

Validation:

• rg -n -e 'git clone https://github.com/bitcoin/bitcoin' -e 'cd bitcoin/' -e 'make -C src/ bitcoind' -e 'Fuzzing Bitcoin Core using' -e 'quickly get started fuzzing Bitcoin Core' doc/fuzzing.md
• git diff --check HEAD~1..HEAD

Bounty:

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