[Serverless] Fixes serverless references

[jmikell821]

Addresses part of elastic/security-docs#6026 (from our serverless bug bash), mostly in the general serverless content set.

Fixes serverless references + a few other slight nits I noticed while browsing.

Attributes:

:serverless-full:  Elastic Cloud Serverless
:serverless-short: Serverless
:es-serverless:    Elasticsearch Serverless
:es3:              {es-serverless}
:obs-serverless:   Elastic Observability Serverless
:sec-serverless:   Elastic Security Serverless
:serverless-docs:  https://docs.elastic.co/serverless

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[lcawl]

LGTM