tweak billing pricing model section structure

[shainaraskas]

elastic.co/docs/deploy-manage/cloud-organization/billing#pricing-model

++ good idea to use the subsection on that page!

I'm not sure how to add changes to another file to this PR, so I'll just describe it here:
I think that page would be even more effective if it was laid out this way, because of the ways eyes scan the page, and how hyperlinks stand out.

Each Elastic Cloud Serverless project type also has its own billing dimensions.

    [Elastic Cloud Hosted deployments billing dimensions](https://www.elastic.co/docs/deploy-manage/cloud-organization/billing/cloud-hosted-deployment-billing-dimensions)
    [Elastic Cloud Serverless billing dimensions](https://www.elastic.co/docs/deploy-manage/cloud-organization/billing/serverless-project-billing-dimensions)
        [Elasticsearch projects billing dimensions](https://www.elastic.co/docs/deploy-manage/cloud-organization/billing/elasticsearch-billing-dimensions)
        [Elastic Observability projects billing dimensions](https://www.elastic.co/docs/deploy-manage/cloud-organization/billing/elastic-observability-billing-dimensions)
        [Elastic Security projects](https://www.elastic.co/docs/deploy-manage/cloud-organization/billing/security-billing-dimensions)

Originally posted by @demers-eric in #2596 (comment)