Additional cleanup for error-prone descriptions

[Philzen]

Following the discussion in https://github.com/openrewrite/rewrite-docs/pull/282

The error-prone template descriptions by picnic seem to need some more treatment. See this example for https://docs.openrewrite.org/recipes/tech/picnic/errorprone/refasterrules/testngtoassertjrulesrecipes:

TODOs:

• remove leading and trailing underscore
• remove any <p> tags
• convert the <pre>-block to a markdown code block and remove the surrounding {@code … } custom tag

https://github.com/openrewrite/rewrite-docs/pull/282/commits/5925d43b8d3816a6df73ad9b820b49425f1807e8 sort of already describes a good test case.

[timtebeek]

Thanks for logging the issue separately; providing the context & links from the other issue for quick reference:

• The JavaDoc for the original Refaster rule
• This JavaDoc is then extracted into a recipe description with some light modifications already https://github.com/openrewrite/rewrite-templating/blob/7eceffca8aee1e899b2614555dc938083538a20c/src/main/java/org/openrewrite/java/template/processor/RefasterTemplateProcessor.java#L439-L461
• The description is converted into Markdown in this generator, which should already account for the _ 🤔

All in all this will require some fixes both here and in the markdown generator, before they show up as fixed in the docs.