Fix issues reported by linkcheck

[rffontenelle]

See the log of linkcheck job. Specifically:

( extension: line 570) redirect https://github.com/KhronosGroup/Vulkan-Guide/blob/master/chapters/layers.md - with Found to https://github.com/KhronosGroup/Vulkan-Guide/blob/main/chapters/layers.md
( flatpak-devel: line 368) broken https://www.electron.build/configuration/flatpak.html - 404 Client Error: Not Found for url: https://www.electron.build/configuration/flatpak.html

[rffontenelle]

Temporary issues came up when running the check, a re-run should green it up.

( extension: line 474) broken https://hg.mozilla.org/mozilla-central/diff/59e57f57dcb73a286822276d02f16e7b17018de6/taskcluster/docker/firefox-flatpak/runme.sh - HTTPSConnectionPool(host='hg.mozilla.org', port=443): Max retries exceeded with url: /mozilla-central/diff/59e57f57dcb73a286822276d02f16e7b17018de6/taskcluster/docker/firefox-flatpak/runme.sh (Caused by SSLError(SSLEOFError(8, '[SSL: UNEXPECTED_EOF_WHILE_READING] EOF occurred in violation of protocol (_ssl.c:1007)')))
( extension: line 490) broken https://hg.mozilla.org/mozilla-central/file/23ee4ac2d048de0aac3fa27ce7eb0925c1903096/xpcom/io/SpecialSystemDirectory.cpp#l198 - HTTPSConnectionPool(host='hg.mozilla.org', port=443): Max retries exceeded with url: /mozilla-central/file/23ee4ac2d048de0aac3fa27ce7eb0925c1903096/xpcom/io/SpecialSystemDirectory.cpp (Caused by SSLError(SSLEOFError(8, '[SSL: UNEXPECTED_EOF_WHILE_READING] EOF occurred in violation of protocol (_ssl.c:1007)')))