Update Pages and Data Fetching docs for SSG improvements (#10837)

* Update pages doc for SSG

* Add data fetching doc

* Format code

* Link directly to nextjs.org

* Fix link

* Add .md

* Add .md

* Fix hashes

* Update pages.md

* Wordsmith

* Fix link

* Fix heading

* Capitalization

* Link to pages

* Add router link

* Fix parens

* Make links consistent

* Use commas

* Link to fallback pages

* Update data-fetching.md

* Fix linting

Co-authored-by: Tim Neutkens <tim@timneutkens.nl>