[website] Rename bcha section names

Summary: BCHA name is obsolete

Test Plan:
before and after change do git grep bcha-, git grep ecash-, git grep bchapage and git grep ecashpage
Built and ran site locally to see that it looks fine

Depends on D10212

Reviewers: #bitcoin_abc, bytesofman

Reviewed By: #bitcoin_abc, bytesofman

Differential Revision: https://reviews.bitcoinabc.org/D10214