[#12022] [#10623] Update development, troubleshooting, and process documentation

[wkurniawan07]

Fixes #12022, also fixes #10623.

Outline of Solution

Most changes can be derived from the commit messages and original issue description.

• The default index.html is added to resolve the frequently asked localhost:8080 returns 404/403. This gets overridden if there is an existing front-end bundle.
• The JDK 17 lint and component tests are only run after pushing to master, at least for now.
• Aside from the two Powerpoint diagrams finally converted to PUML, there are some parts of the documentation still using raw image even though the PUML counterpart exists. All are now standardized.
  • Also, some parts of the design diagrams are outdated and referring to pre-V8 version (e.g. GaeSimulation, GAE Remote API).

We should not need to preserve the issues labelled with c.Message or a-DevHelp to discussions, as almost all of them are already outdated (pre-V8, pre-V7, IDE troubleshooting, etc.) and the few ones still relevant are already covered in the updated troubleshooting guide as well as existing discussions.

[wkurniawan07]

@samuelfangjw @zhaojj2209 feel free to add more process-related updates to the same branch

[samuelfangjw]

Happy with the changes, thanks for updating the docs!

[zhaojj2209]

LGTM!