Various docs point at the old couchdb.org website

Various docs point at the old couchdb.org website. This has been deprecated, so some/all of these references should be updated to corresponding content on https://couchdb.apache.org/.

References:

```
$ git grep -Fl couchdb.org
TESTING.md
docs/_guides/mango-queries.md
docs/_guides/replication.md
docs/_guides/setup-couchdb.md
docs/_includes/api/batch_create.html
docs/_includes/api/batch_fetch.html
docs/_includes/api/changes.html
docs/_includes/api/fetch_document.html
docs/_includes/api/query_database.html
docs/_includes/api/query_index.html
docs/_includes/api/replication.html
docs/_includes/api/save_attachment.html
docs/_posts/2014-04-14-pagination-strategies-with-pouchdb.md
docs/_posts/2014-05-01-secondary-indexes-have-landed-in-pouchdb.md
docs/_posts/2014-06-17-12-pro-tips-for-better-code-with-pouchdb.md
docs/_posts/2015-04-05-filtered-replication.md
docs/_posts/2016-12-12-pouchdb-6.1.0.md
docs/_posts/2017-04-20-pouchdb-6.2.0.md
docs/_posts/2022-04-13-pouchdb-7.3.0.md
packages/node_modules/pouchdb-find/README.md
packages/node_modules/pouchdb-merge/src/winningRev.js
tests/integration/deps/pouchdb-1.1.0-postfixed.js
tests/integration/deps/pouchdb-2.0.0-postfixed.js
tests/integration/deps/pouchdb-2.2.0-postfixed.js
tests/integration/deps/pouchdb-3.0.6-postfixed.js
```