Base: Add man pages for null/zero/full character devices

Base/usr/share/man/man4/full.md

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+## Name
+
+full - always full device
+
+## Description
+
+`/dev/full` is a character device which is always full.
+
+Reading from `/dev/full` returns '\0' bytes and exits with status 0.
+
+Writing to `/dev/full` fails with ENOSPC error.
+
+To create it manually:
+
+```sh
+mknod /dev/full c 1 7
+chmod 666 /dev/full
+```
+
+## Files
+
+* /dev/full
+
+## Examples
+
+```sh
+$ head -c 8 /dev/full | hexdump
+00 00 00 00 00 00 00 00
+```
+
+## See also
+
+* [`null`(4)](../man4/null.md)
+* [`zero`(4)](../man4/zero.md)
+

Base/usr/share/man/man4/null.md

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+## Name
+
+null - data sink
+
+## Description
+
+`/dev/null` is a character device file which discards input.
+
+Reading from `/dev/null` returns end of file and exits with status 0.
+
+## Files
+
+* /dev/null
+
+## See also
+
+* [`full`(4)](../man4/full.md)
+* [`zero`(4)](../man4/zero.md)
+

Base/usr/share/man/man4/zero.md

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+## Name
+
+zero - data sink
+
+## Description
+
+`/dev/zero` is a character device file which discards input.
+
+Reading from `/dev/zero` returns '\0' bytes and exits with status 0.
+
+## Files
+
+* /dev/zero
+
+## Examples
+
+```sh
+$ head -c 8 /dev/zero | hexdump
+00 00 00 00 00 00 00 00
+```
+
+## See also
+
+* [`null`(4)](../man4/null.md)
+* [`full`(4)](../man4/full.md)
+