[website] Rename Announcements to Blog

Summary:
Blog is more descriptive of what it is.
Doesn't affect external links to the posts.

Depends on D10997

Test Plan:
Ran it locally, checked links and pages work properly.
Did git grep announcements

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Maniphest Tasks: T1919

Differential Revision: https://reviews.bitcoinabc.org/D10998