dev-lua/lgi: fix dohtml use in src_install(), bug #694958

dev-lua/lgi: fix dohtml use in src_install(), bug #694958

Ebuild failed at src_install() as:
```
 * ERROR: dev-lua/lgi-0.9.0-r1::gentoo failed (install phase):
 *   'dohtml' has been banned for EAPI '7'
```

Switched to the direct 'dodoc'. While at it disabled compression
for provided samples and markdown docs.

Reported-by: dashmz@web.de
Closes: https://bugs.gentoo.org/694958
Package-Manager: Portage-2.3.76, Repoman-2.3.17
Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>