feat: Split up Android page and add index page for Android recommendations

[redoomed1]

Changes proposed in this PR:

• Split up Android page into separate pages and add an Android index page (full proposal here)
  • The chief benefit of this change is that it would allow for more detailed entries for each section without worrying about making the Android page too long. For example, Jonah, on the forum, floated the idea of refreshing the custom Android operating system sections so that they provide more substantial explanations.
  • No major changes to the content ported from the original Android page
  • Updated link to AOSP source code
  • Added internal link to Google Pixel card in the description under the GrapheneOS card
  • Changed GrapheneOS docs links under the DivestOS card to more specific links
  • Changed external links to internal ones in the GrapheneOS App Store section
  • Changes for the new pages
  • Added introductions for the "Obtaining Android Apps" and "Mobile Phones" pages
  • Updated image file paths and internal links
  • Changed admonition about EOL devices from Note to Warning on the Mobile Phones page
  • Added buttons to the intro to the Mobile Phones page that point to the Custom Android OSes page and the Android Overview page
• Modify tools.md to reflect the above changes
• Delete original Android page

Closes #2611

To Do

All done!

- [x] Fix images and internal links in all newly created pages - [x] Fix internal links outside of the Android section that point to the original Android page - [x] Add introductions for the Obtaining Android Apps page and the Mobile Phones page - [x] Modify `tools.md` to reflect the above changes

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- [x] I agree to the terms listed below:

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