add link validation

.gitignore

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 .idea
-.vscode/
+.vscode/
+node_modules

wiki-tools/HOWTO-validate-links.md

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+# link validation in markdown documents
+
+VS Code introduced a [link validator](https://code.visualstudio.com/Docs/languages/markdown) in version 1.72. This seems the best approach while a single file is edited.
+
+For bulk validation <https://github.com/webhintio/markdown-link-validator> seems best
+
+## findings
+
+- debug mode is extremely helpful for troubleshooting
+- it's incredible fast
+- should work cross-platform (tried on Windows & WSL Debian)
+- <https://www.te.com/usa-en/> seems to block or throttle access via an automated script after a certain number of requests
+
+```bash
+  markdown-link-validator:utils:request problem with request: connect ETIMEDOUT 2a01:4f8:221:3b0a:1000:0:19:200:443 - https://www.lammertbies.nl/comm/cable/RS-232.html +21s
+  markdown-link-validator:utils:request problem with request: read ECONNRESET - https://www.te.com/usa-en/product-4-1437290-1.html +1m
+```
+
+## solution
+
+- use exclude list for batch validation, skip all external links
+
+```powershell
+markdown-link-validator . -i https?:\/\/.* --debug >.\wiki-tools\md_link_validation.log
+...
+Found a total of 1361 links in directory "C:\Users\mholzer\Documents\git\rusefi_documentation":
+    1298 valid
+    63 invalid
+Time to validate: 1.07s
+```
+
+## open issue(s)
+
+- would like to have only "invalid" references in output. There is [already an enhancement request](https://github.com/webhintio/markdown-link-validator/issues/6) filed since Aug 2019 :(
+- requested to [assign committer](https://github.com/webhintio/hint/issues/5382)
+
+## workaround
+
+<https://github.com/chmac/markdown-link-validator/commits/omit-success-output> has a quick fix
+
+to install:
+note: as of 2022-12-13 still in testing with some issues!!
+
+```powershell
+npm install --save chmac/markdown-link-validator#omit-success-output
+```
+
+to run:
+
+```powershell
+node .\node_modules\markdown-link-validator\dist\src\bin\markdown-link-validator.js . -i https?:\/\/.* --debug
+```