Add methods to JavadocComment to refactor how the Client Overview table is created

[alicejli]

The Client Overview table in https://github.com/googleapis/sdk-platform-java/blob/main/gapic-generator-java/src/main/java/com/google/api/generator/gapic/composer/comment/ServiceClientCommentComposer.java#L221 has been brittle in terms of ensuring proper formatting of all of its elements. (See #2280 and #2361 as piecemeal fixes)

It would be better to refactor this table to ensure all elements are always properly formatted. Likely some new methods will need to be added to JavaDocComment.