Discuss: Remove pages from docs

Per these issues

• https://github.com/expressjs/discussions/issues/211
• https://github.com/expressjs/discussions/issues/105

Let's remove these ancillary pages. This will also enable us to close a number of open issues and PRs.

In the "Resources" menu:

• Template engines * (NOTE: This is in addition to the "Guide > Using template engines" and "Advanced topics > Developing template engines" topics)
• Companies using Express
• Open source projects using Express *
• Additional learning * NOTE: THIS IS LOCALIZED
• Frameworks built on Express *

And these under the "Advanced" menu:

• Process managers * NOTE: THIS IS LOCALIZED
• Health Checks and Graceful Shutdown > Third-party solutions

* These are marked with this warning:

Warning: This information refers to third-party sites, products, or modules that are not maintained by the Expressjs team. Listing here does not constitute an endorsement or recommendation from the Expressjs project team.

How to do it

Remove these files:

en dir:

• resources/template-engines.md
• resources/companies-using-express.md
• resources/open-source-using-express.md
• resources/learning.md
• resources/frameworks.md
• advanced/pm.md

6 files total

All other lang dirs:

• advanced/pm.md
• resources/learning.md

2 * 15 langs = 30 files total

Grand total of 36 files removed.

Keeping the content?

We could move the content to this wiki. Pros/cons?