Add docs/users_guide/.log to .gitignore

When the users guide fails to build (as in #17346), a
`docs/users_guide/.log` file will be generated with contents that
look something like this:

```
WARNING: unknown config value 'latex_paper_size' in override, ignoring
/home/rgscott/Software/ghc5/docs/users_guide/ghci.rst:3410: WARNING: u'ghc-flag' reference target not found: -pgmo ?option?
/home/rgscott/Software/ghc5/docs/users_guide/ghci.rst:3410: WARNING: u'ghc-flag' reference target not found: -pgmo ?port?

Encoding error:
'ascii' codec can't encode character u'\u27e8' in position 132: ordinal not in range(128)
The full traceback has been saved in /tmp/sphinx-err-rDF2LX.log, if you want to report the issue to the developers.
```

This definitely should not be checked in to version control, so let's
add this to `.gitignore`.
21 jobs for master in 110 minutes and 16 seconds (queued for 5 seconds)
Status Job ID Name Coverage
  Lint
passed #177828
lint
lint-linters

00:00:14

passed #177831
lint
lint-submods-branch

00:00:15

passed #177829
lint
lint-testsuite

00:00:15

passed #177830
lint
typecheck-testsuite

00:00:13

 
  Build
passed #177833
x86_64-linux
hadrian-ghc-in-ghci

00:17:07

passed #177835
x86_64-linux
release-x86_64-linux-deb9-dwarf

01:19:35

failed #177834
x86_64-linux
validate-x86_64-linux-deb9-debug

01:49:31

passed #177832
x86_64-linux
validate-x86_64-linux-deb9-hadrian

01:48:21

 
  Full Build
skipped #177837
aarch64-linux allowed to fail
validate-aarch64-linux-deb9
skipped #177838
x86_64-linux
validate-i386-linux-deb9
skipped #177836
x86_64-darwin
validate-x86_64-darwin
skipped #177839
x86_64-linux
validate-x86_64-linux-deb9-integer-simple
skipped #177840
x86_64-linux
validate-x86_64-linux-deb9-unreg
skipped #177841
x86_64-linux
validate-x86_64-linux-fedora27
skipped #177843
x86_64-windows allowed to fail
validate-x86_64-windows
skipped #177842
x86_64-windows allowed to fail
validate-x86_64-windows-hadrian
 
  Cleanup
passed #177844
x86_64-darwin
cleanup-darwin

00:00:10

 
  Packaging
skipped #177845
x86_64-linux allowed to fail
doc-tarball
 
  Testing
skipped #177846
x86_64-linux allowed to fail manual
hackage
skipped #177847
x86_64-linux
perf-nofib
 
  Deploy
skipped #177848
x86_64-linux allowed to fail
pages
 
Name Stage Failure
failed
validate-x86_64-linux-deb9-debug Build
$ cp -Rf $HOME/.cabal cabal-cache
Uploading artifacts...
ghc-*.tar.xz: found 1 matching files
junit.xml: found 1 matching files

Uploading artifacts to coordinator... ok
id=177834 responseStatus=201 Created token=fUmN2ix9
Uploading artifacts...
junit.xml: found 1 matching files

Uploading artifacts to coordinator... ok
id=177834 responseStatus=201 Created token=fUmN2ix9
ERROR: Job failed: exit code 1