Fix benchmark file links in Appendix 4

[mmustafasenoglu]

Fixes #5126

The benchmark file names in the "Files used" table were written as interpreted text instead of references, so the link targets at the bottom of docs/app4.rst were never used and the table rendered without any download links. The artifex.com sample URLs also return 404 now, so the targets now point to the archived copies at web.archive.org (each archived file's size matches the size column in the table).

Changes:

• docs/app4.rst: added the trailing _ to the eight file names so they reference the existing targets
• docs/app4.rst: pointed the eight targets at their web.archive.org copies
• docs/app4.rst: added a note that the original artifex.com downloads are gone

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[mmustafasenoglu]

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