Include btop manpage

Description:

Btop as of release 1.4.0 now ships a man page (btop.1).

This man page is not currently present in the Arch package of Btop.
According to the upstream makefile lowdown will be required as a new compile time dependency
to translate the manpage.md file.
https://github.com/aristocratos/btop/blob/v1.4.0/Makefile#L305-L312

Additional info:

• package version(s): 1.4.0-1 and 1.4.0-2
• config and/or log files: n/a
• link to upstream bug report, if any: https://github.com/aristocratos/btop/pull/724

Steps to reproduce:

1. sudo pacman -Syu btop
2. man btop
3. man: No entry for btop in the manual.