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

[Felix Singer (Code Review)]

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

Felix Singer 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:

(3 comments)

Patchset:

PS1: 
Great idea!

File Documentation/external_docs.md:

https://review.coreboot.org/c/coreboot/+/70239/comment/58dd54ca_a17c29e3 
PS1, Line 58: * Intel
`[Architecture Software Developer Manuals](https://www.intel.com/content/www/us/en/developer/articles/technical/intel-s...

https://review.coreboot.org/c/coreboot/+/70239/comment/3a978862_76b0f7b4 
PS1, Line 68:  * [IOTools](https://github.com/adurbin/iotools) - Command line tools to access hardware registers
```
[UEFITool](https://github.com/LongSoft/UEFITool)
[CHIPSEC](https://chipsec.github.io)
```