Add redirects for relocated topics

[infotexture]

With the new top-level structure of the documentation for dita-ot/docs#121, the file system locations of many topics will change between 3.0 & 3.0.1 when dita-ot/docs#185 is merged with the hotfix/3.0.1 branch.

Although few deep links to 3.0 topics are likely to have been shared so soon after the initial release, several have already been sent via email and other channels, so redirects should be set up to automatically load (at least these) topics from their new locations.

GitHub Pages does not support redirecting via server configuration files such as .htaccess or .conf, but recent versions of the gem do support redirects via the jekyll-redirect-from plugin, which serves HTML files with an HTTP-REFRESH meta tag that points to the new destination.

As the name suggests, the plugin’s default usage is to add redirect_from metadata to the YAML header of files that should answer to alternative URLs, so the file that includes these entries is loaded instead. This approach is less useful in our case, where files are auto-generated from a common layout template, so individual overrides are less practical.

However, the plugin also supports a lesser-known redirect_to mode that can be used to set up (otherwise empty) pages at the old location with a YAML front-matter block that points to the new location:

---
# Redirect from old permalink
layout: base
title: Moved to new location…
redirect_to:
  - /3.0/topics/migration.html
---

I have verified this approach in local testing, and plan to roll out the changes with the 3.0.1 hotfix.

[infotexture]

Still need to verify which 3.0 URLs have been shared thus far and set up the redirects for these files.

I don’t think it’s necessary to set up redirects for every relocated topic, so will check analytics data and forums for clues on which URLs need to be redirected.

[infotexture]

The following 3.0 URLs have been publicly shared and will need to be redirected for 3.0.1:

• http://www.dita-ot.org/3.0/dev_ref/migration.html
  → /3.0/topics/migration.html
• http://www.dita-ot.org/3.0/dev_ref/plugin-newtranstype.html
  → /3.0/topics/plugin-newtranstype.html
• http://www.dita-ot.org/3.0/dev_ref/html-customization-plugin-bundle-css.html
  → /3.0/topics/html-customization-plugin-bundle-css.html

[infotexture]

The changes will of course also affect paths in the dev docs.

While these are by nature more volatile and never promised to be stable, we might consider redirecting a few dev URLs that have been recently shared or receive above-average traffic, such as:

• http://www.dita-ot.org/dev/dev_ref/migration.html
  → /dev/topics/migration.html
• http://www.dita-ot.org/dev/dev_ref/plugin-addgeneratedtext.html
  → /dev/topics/plugin-addgeneratedtext.html
• http://www.dita-ot.org/dev/user-guide/build-migrating-ant-to-dita.html
  → /dev/topics/migrating-ant-to-dita.html
• http://www.dita-ot.org/dev/dev_ref/pdf-customization-best-practices.html
  → /dev/topics/pdf-customization-best-practices.html