Updates to Activity docs (#7884)

* Updates to Activity docs

* wip

* Updates to Activity docs:

- New intro at top of Reference
- Adds "Deferring hydration of low-priority content" section
- Move "Preventing hidden content from having unwanted side effects" section to Troubleshooting
- Removes SSR section from Troubleshooting

* useLayoutEffect + Tidying up

* Use the same color code step for "visible"

* Tidy

* Typo

* Update section on selective hydration

* Update title

* Clean up props

* Tweak wording

* wip

* wip

* wip

* remove temporary scroll restoration while working on docs

* wip

* wip

Author: samselikoff