DOC-3711: Simplify Antora content source root

[eric-schneider]

Jira: DOC-3711

This PR moves antora.yml and the modules directory to the root of the repository.

Important

The draft preview on this PR will never succeed because it can't pick up the changes in https://github.com/riptano/datastax-docs-site/pull/295. When both PRs are merged at the same time, the prod build will succeed.

[plpesvc-ds]

Build failure! ❌

[04:51:07.371] FATAL (antora): the start path 'docs-src/pulsar-sink-core' does not exist in https://github.com/datastax/pulsar-sink-docs.git (branch: DOC-3711-batch1) Cause: Error at /home/runner/work/datastax-docs-site/datastax-docs-site/node_modules/@antora/content-aggregator/lib/aggregate-content.js:559:11 at async collectFilesFromStartPaths (/home/runner/work/datastax-docs-site/datastax-docs-site/node_modules/@antora/content-aggregator/lib/aggregate-content.js:452:18) at async Promise.all (index 2) at async Promise.all (index 0) at async generateSite (/home/runner/work/datastax-docs-site/datastax-docs-site/node_modules/@antora/site-generator/lib/generate-site.js:25:5) at async Command.parseAsync (/home/runner/work/datastax-docs-site/datastax-docs-site/node_modules/commander/lib/command.js:1092:5)