[M] Change in coreboot[master]: Docs: Add page for external resources

[Nicholas Chin (Code Review)]

Attention is currently required from: Martin L Roth, Maximilian Brune.

Nicholas Chin has posted comments on this change. ( https://review.coreboot.org/c/coreboot/+/70239 )

Change subject: Docs: Add page for external resources
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Patch Set 1:

(1 comment)

File Documentation/external_docs.md:

https://review.coreboot.org/c/coreboot/+/70239/comment/bfc0c4b2_8a19ec7a 
PS1, Line 11: ## General Information
```
* [Open Security Training](https://opensecuritytraining.info/Training.html),
  [Open Security Training 2](https://p.ost2.fyi/):
  Open course material for a variety of topics such as assembly, firmware, security, debugging, and more.
```