Document method for sitemap pregen and caching

### Description

There is a lack of clear guidance on how to generate sitemaps in a way that does not impact environment stability.

By default, sitemap functionality (namely from Yoast), can impact application performance if crawlers or bad actors make many requests. To offset this, sitemaps can be pre-generated, and be cached.

We should write clear guidance on how to approach this.

### Acceptance Criteria
- [ ] A new Altis documentation page which describes the most efficient method to generate sitemaps without impacting environment stability.
- [ ] The Gotchas regarding sitemap generation especially for sites which a large number of posts should be added.
- [ ] Verify that our advice "works" before publishing.

## For Altis Team Use

### Ready for Work Checklist

Is this ticket ready to be worked on? See
[the Play Book Definition of Ready](https://playbook.hmn.md/play/product/definition-of-ready/)

- [x] Is the title clear?
- [x] Is the description clear and detailed enough?
- [x] Are acceptance criteria listed?
- [x] Have any dependencies been identified? (Optional)
- [x] Have any documentation/playbook changes been identified? (Optional)
- [x] Is an estimate or time box assigned?
- [x] Is a priority label assigned?
- [ ] Is this ticket added to a milestone?
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### Completion Checklist

Is this ticket done? See
[the Play Book Definition of Done](https://playbook.hmn.md/play/product/definition-of-done-2/)

- [ ] Has the acceptance criteria been met?
- [ ] Is the documentation updated (including README)?
- [ ] Do any code/documentation changes meet project standards?
- [ ] Are automatic tests in place to verify the fix or new functionality?
- [ ] Or are manual tests documented (at least on this ticket)?
- [ ] Are any Playbook/Handbook pages updated?
- [ ] Has a new module release (patch/minor) been created/scheduled?
- [ ] Have the appropriate `backport` labels been added to the PR?
- [ ] Is there a roll-out (and roll-back) plan if required?