Fix broken A/B boot update cross-reference; remove leftover orphaned source files

[ZayanKhan-12]

Six commits, intended to be cherry-pickable individually:

1. Fix broken cross-reference — computers/raspberry-pi/boot-eeprom.adoc linked xref:../services/connect.adoc#ab-boot-update, but no such id exists anywhere in the Connect page: the target section declares the explicit anchor [[remoteupdate-ab]] (and its title derives a-b-boot-update). The link therefore landed at the top of the Connect page instead of the A/B Boot Update section. Fixed to the stable explicit anchor.
   2–5. Remove four empty leftover files — computers/configuration/boot_folder.adoc, device-tree.adoc, pin-configuration.adoc (all emptied and unreferenced since the configuration restructure, Update linux.md #418) and accessories/display/legacy.adoc (since the Touch Display consolidation, add a note to the expand-filesystem option #470). All are zero-byte and included by nothing.
2. Remove orphaned computers/configuration/use-a-proxy.adoc — unreferenced by any page or manifest since the configuration clean-up; its content is superseded by the "Configure your Raspberry Pi to use a proxy server" section ([[proxy]]) in configuration/configuring-networking.adoc. Happy to drop this commit if you'd prefer to keep the file around.

Verification (macOS, Ruby 3.2 + ninja, matching CI): full make html build succeeds; the built computers/raspberry-pi.html now links services/connect.html#remoteupdate-ab and that id exists in the built Connect page (the old #ab-boot-update href is gone); no built page references the removed files. The only asciidoctor diagnostic in the build log (a table-row warning in accessories/usb/about.adoc) is pre-existing on master and untouched by this PR.

Prepared with AI assistance (Claude); every reference was mechanically verified against the source tree and the built site as described above.

[katshann-raspberry]

@ZayanKhan-12 Thank you for your contribution. We'll work on publishing it soon.