Create Jira issues parser basic classes

We need to write a reliable parser that converts Jira markwdown to Gitlab markdown. We can't do it with simple regexps as it is not reliable. This issue is intended as a start issues for implementing a such a parser.

Before starting with the implementation we need to decide what approach we'll use, we need to choose the gem we'll build it on.

After we discuss that we should add the gem and create a basic class & specs that does nothing other than takes the input and returns output without any further changes to the gem such as adding new tags, validating, parsing links etc.

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Assuming we can use the Jira V3 API to get text fields (like description), then we can use the Atlassian Document Format (#212295 (comment 341265335))

Various ADF links:

• Atlassian Document Format
• Document Builder
• Document Viewer
• Jira V3 REST API
• The Atlassian Document Format is Coming to an API Near You

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