Fix owncloud.org history and created a missing file

[mmattel]

Link corrections in the page footer:

• owncloud.org/history is no longer available and there is no replacement for - line deleted
• other available links pointing to a new name have been corrected

Missing file:

• release_notes.adoc was missing
  
  The file contains now a basic content which must be updated on new doc releases.

Fixes: #200 (https://owncloud.org/history is no longer a valid link)

Update:
After digging and drilling why the changes like history did not ave any effect, I found the solution.
If you change any of the .hbs files, you also need to add/replace them in ui-bundle.zip. Only then the changes take effect !

@settermjd @hurradieweltgehtunter

[tboerger]

The ui bundle should be downloaded from the doc theme repo.

[mmattel]

Just for completeness, ui-bundle.zip cant currently be found neither in docs-themes or docs-ui

[settermjd]

Just for clarity, the ui-bundle.zip isn't tracked in the repo, but one's available as a build artifact with both releases. To see the changes, you need to first rebuild the theme after you make any changes, using gulp pack, then you need to point the docs repo at the newly generated ui-bundle.zip, so that the changes are visible. I didn't track ui-bundle.zip, as it didn't make sense when it's a build artifact (at least it didn't make sense to me).

[settermjd]

I think I need to turn that last comment into a blog post.