Meta: Elastic Endpoint and Elastic Agent doc revamp

[jmikell821]

Using this to track changes required for our endpoint/agent docs, which will be done in stages.

### Tasks
- [x] Add system requirements - CPU, memory, disk space (will be cross-platform for all OSes)
- [ ] https://github.com/elastic/security-docs/issues/4636
- [ ] https://github.com/elastic/security-docs/pull/5749
- [ ] https://github.com/elastic/security-docs/issues/5793
- [ ] https://github.com/elastic/security-docs/issues/5792
- [ ] https://github.com/elastic/security-docs/issues/5791