Introduction: Add figure and table captions

[kito99]

Right now the tables and figures don't have captions in the Overview. (Compare the images in the overview https://jakartaee.github.io/jakartaee-documentation/jakartaee-tutorial/current/intro/overview/overview.html with the ones in the Security chapter: https://jakartaee.github.io/jakartaee-documentation/jakartaee-tutorial/current/security/security.html).

To fix this, look at how table and figure captions are done in the Security chapter and elsewhere, and link to them as we do elsewhere in the tutorial. Relevant paths in the repo:

• Overview: src/main/antora/modules/intro/pages/overview/overview.adoc
• Security: src/main/antora/modules/security/pages/security.adoc

Also, update the Contributing guide accordingly to explain how to add captions for figures and tables.

[kito99]

@jhammen this is a possibility for you.