doc: fix broken links

[alexandear]

The PR fixes broken links on https://perkeep.org/ found by the linkcheck.

linkcheck output

❯ linkcheck https://perkeep.org/
Done crawling.

https://perkeep.org/doc/environment-vars
- (69:17) 'pkg/osutil' => https://perkeep.org/pkg/osutil (HTTP 404)
- (112:14) 'pkg/osutil' => https://perkeep.org/pkg/osutil (HTTP 404)
- (170:17) 'pkg/osutil' => https://perkeep.org/pkg/osutil (HTTP 404)
- (193:30) 'pkg/images' => https://perkeep.org/pkg/images (HTTP 404)

https://perkeep.org/doc/prior-art
- (60:13) 'terminol..' => https://perkeep.org/doc/terms.md#claims (HTTP 200 but missing anchor)
  - redirect path:
    - https://perkeep.org/doc/terms.md (302)
    - /doc/terms (200)

https://perkeep.org/doc/release/0.2
- (64:73) 'camput' => https://perkeep.org/cmd/camput (HTTP 404)

https://perkeep.org/doc/release/monthly/2017-03-01
- (54:62) 'Scanning..' => https://perkeep.org/app/scanningcabinet (HTTP 404)

https://perkeep.org/doc/release/monthly/2017-04-05
- (56:0) 'scanning..' => https://perkeep.org/app/scanningcabinet (HTTP 404)

https://perkeep.org/doc/release/monthly/2017-05-05
- (52:15) 'launcher' => https://perkeep.org/launch (HTTP 301 => 500)
  - redirect path:
    - https://perkeep.org/launch (301)
    - /launch/ (500)

https://perkeep.org/doc/terms
- (153:55) 'pk put' => https://perkeep.org/cmd/pk%20put (HTTP 404)
- (153:55) 'pk put' => https://perkeep.org/cmd/pk%20put (HTTP 404)

Errors. Checked 28018 links, 1615 destination URLs (206 ignored), 6 have errors, 1 has warning(s).