Add Thymeleaf sample

[0xkookoo]

Thymeleaf is a modern server-side Java template engine for both web and standalone environments.
I develop a very starting-based guide to implement Thymeleaf in Spring Boot Web as a sample.
This is the first time I contribute to Spring, so if there is any layout or typesetting problems, please let me know. Thanks for all the contributors and maintainers of Spring Boot!

[pivotal-issuemaster]

@Lisanaaa Please sign the Contributor License Agreement!

Click here to manually synchronize the status of this Pull Request.

See the FAQ for frequently asked questions.

[pivotal-issuemaster]

@Lisanaaa Thank you for signing the Contributor License Agreement!

[wilkinsona]

Thanks for opening your first Spring Boot pull request. We already have a number of samples that use Thymeleaf, but their names don't make it clear that they do. Of those samples, spring-boot-sample-web-ui is the one that's most focussed on Thymeleaf. Rather than adding another sample, I think we could just rename that one.

Labeling for team attention to see what the rest of the team think.

[0xkookoo]

I download the spring-boot-sample-web-ui and run it. But I got some thing wrong,

org.thymeleaf.exceptions.TemplateProcessingException: Could not parse as fragment selection: "fragments :: head(title=~{::title/text()})" (messages/list:3)
	at org.thymeleaf.standard.expression.FragmentSelectionUtils.parseFragmentSelection(FragmentSelectionUtils.java:106)
	at org.thymeleaf.standard.fragment.StandardFragmentProcessor.computeStandardFragmentSpec(StandardFragmentProcessor.java:73)
	at org.thymeleaf.standard.processor.attr.AbstractStandardFragmentHandlingAttrProcessor.computeFragment(AbstractStandardFragmentHandlingAttrProcessor.java:68)

And I think a more clear and easier guide is better for the newbies. No matter what's the next step, I will still contribute. Be in this Spring family.

[philwebb]

And I think a more clear and easier guide is better for the newbies

Our samples are somewhat misnamed, we mainly use them to test the integrity of the release. Perhaps we should look at adding a dedicated Thymeleaf sample to spring.io/guides. The repo for that project is https://github.com/spring-guides

[wilkinsona]

There's an existing Spring MVC and Thymeleaf guide.

[wilkinsona]

We discussed this yesterday, and decided that what's being proposed here is already covered by the existing guide. I've opened spring-guides/gs-serving-web-content#24 to hopefully make it a bit clearer that the guide uses Thymeleaf. We've decided not to rename our existing web UI sample as it doesn't focus specifically on Thymeleaf but also uses Bootstrap etc. Thanks anyway.