nixos/doc: improve install instructions

- Update download URLs
- Replace "USB stick"/"USB Drive" with "USB flash drive" as that seem more correct

  https://en.wikipedia.org/wiki/USB_flash_drive
  https://elementary.io/docs/installation#choose-operating-system

- Don't mention CD as easiest option anymore,
  as all modern systems should be able to boot from USB,
  but many don't have a CD drive. Burning CDs is also usually wasteful as you
  can't burn them again.
- Remove link to NixOS Wiki (Making_the_installation_media) as it is not needed
- Add Etcher and USBImager as graphical tools to create install drive
- Make dd command consistent and use block size of 4 MB for faster flashing
- More consistent text
- Add instructions for "Booting from the install medium"

  Inspired by https://github.com/elementary/website/blob/9a91b0f4956b059db02122fbc738c4c067e033a7/docs/installation.md#booting-from-the-install-drive-booting-from-the-installation-medium-clear-float-2

- Add instructions for "Graphical Installation"
- Restructure headings and anchors for "Manual Installation"
- Adding legacy anchors for "Manual Installation" to not break links

Co-authored-by: j-k <dev@j-k.io>
Co-authored-by: Sandro <sandro.jaeckel@gmail.com>
Co-authored-by: Robert Schütz <github@dotlambda.de>
Co-authored-by: Jörg Thalheim <Mic92@users.noreply.github.com>
Co-authored-by: Thiago Kenji Okada <thiagokokada@gmail.com>

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