Include btop manpage
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# Description:
<!-- Describe the bug in full detail. -->
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.<br>
According to the upstream makefile `lowdown` will be required as a new compile time dependency<br>
to translate the `manpage.md` file.<br>
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:
<!-- Describe how to reproduce the bug step by step including the commands -->
1. `sudo pacman -Syu btop`
2. `man btop`
3. *man: No entry for btop in the manual.*
issue