Added functionality to maintain formatting used in XML-docs in generated documentation.

[t-mxcom]

Supported target formats are HTML and Markdown. The methods in XmlDocsExtensions introduce a new parameter which allows the caller to specify the target format. The default value is XmlDocsFormattingMode.Unformatted, which results in exactly the same result as the library worked before.

This pull request is created in response to the issue https://github.com/RicoSuter/NSwag/issues/3422.

It adds support for XML-docs tags <c>...</c>, <b>...</b>, <strong>...</strong> and <u>...</u>. If you like my solution so far, I'd move on to add support for <code>...</code>, <list>...</list> and <para>...</para>.

Additionally, the keywords (names of elements and attributes) of XML-docs are now declared as string constants.

[RicoSuter]

Thanks for the PR. Looks good so far..

Only question for me is whether we should add each config as additional parameter or now introduce a new config object (single parameter) - every time a new parameter is added this is a binary breaking change....

What do you think? /cc @lahma

[RicoSuter]

Lgtm as well, last open point is the name of XmlDocsOptions...

[t-mxcom]

Lgtm as well, last open point is the name of XmlDocsOptions...

see my comment above https://github.com/RicoSuter/Namotion.Reflection/pull/111#discussion_r962259899