doc.php doesn't match with tag target

[fabienrohrer]

When pages with a tag version are displayed, we except to use the corresponding doc.php file.

For example:
https://www.cyberbotics.com/doc/guide/the-user-interface?version=8.6 => https://www.cyberbotics.com/files/repository/www/wwi/8.6/doc.php

This is currently not the case, https://www.cyberbotics.com/files/repository/www/wwi/$(master_version)/doc.php
is used.

This causes difficult backward compatibility issues like these ones:

• Very hotfix: Add fallback for the dependencies.txt mechanism. #798
• Hotfix doc backward compatibility. #819

[fabienrohrer]

Other remark: https://www.cyberbotics.com/files/repository/www/wwi/R2018-rev[1|2]/[doc.php|webots-doc.css|viewer.js] are deployed on the FTP (probably from the deployment script?) and never used.