Upgrade sphinx environment to support linkcheck build goal

[fgdrf]

To continuously check the external links in sphinx docs the build goal linkcheck should run every time the sphinx-build has been started (see root/livedvd/gisvm/trunk/bin/install_main_docs.sh).

​linkcheck is available since version 1.1 of sphinx and check references, which can be configured by set up a ignore list, for example localhost URL's (see linkcheck_ignore config option).

This would help to avoid outdated references to external resources.

[kalxas]

Ticket retargeted after milestone closed

[kalxas]

Ticket retargeted after milestone closed

[kalxas]

Ticket retargeted after milestone closed

[cvvergara]

Adding the link checker Many links are broken some are redirections. ​https://travis-ci.org/github/cvvergara/OSGeoLive-doc/builds/667128348

Pending tasks before making the PR In English documentation

• fix redirections
• fix broken links

In other languages documentation

• update the locale
• push to transifex
• pull the newly untranslated strings (strings with the links will be consider untranslated by transifex but will be 99% similar to a suggestion)

[cvvergara]

Most of the work is done, The current problem to fix is:

have travis pass the job that checks links

• its failing because it is trying to deploy and that build is not for deployment

[cvvergara]

I prepared actions based on the (last) commit message The commit message must have the language code for example [de] updating translations will trigger the German translation link checks.

English is always checked. Because sometimes the link in English fails, which makes the translation fail also. So it can still be merged, but in another PR the English link should be fixed.