[#2122] Add page dedicated to release notes.

Problem
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This started from a comment from @alibotean that we should have ticket id in release notes.


Changes
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I start updating the release notes section from Generic page and found out that release process needs a dedicated page.

For now the release process only contains info about release notes but for the future I plan to add more details about downloadables / known issues / upgrade part / public announcement.

I have updated wiki page to point to this future styleguide page https://trac.chevah.com:10443/wiki/Development/ProductRelease 

I would like to slowly move to a open/public development process and this is why I have published this information on styleguide. Also, the information might be useful for other people working/starting a new project.

For now the wiki will only contain private/technical info or info which are not useful for the public audience.

How to test
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reviewers: @alibotean @lgheorghiu 

Read the new document and let me know if it make sense.

For now styleguide publish / build system is broken. This is why the changes are not published yet. I am working to fix it, but the text can be reviewed.

Thanks!