Add a linter to validate external links

[dshevtsov]

Purpose of this pull request

This pull request (PR):

• Adds a plugin remark-lint-no-dead-urls to ensure that external URLs in Markdown pages are working. The linter returns warnings on https://www.php.net/ on working links in some reason (I assume server related issues) so I excluded this domain in the configuration.
• Replaces or removes dead links

Internal ticket: COMDOX-156

Test

Failed validate job reporting about dead links.

[dshevtsov]

@jeff-matthews please see a report and fix the links.

[jeff-matthews]

Will do @dshevtsov. I'll push directly to this branch unless you say otherwise.

Also, just an FYI that when I run yarn test locally with the new config, I get a bunch of offline warnings:

...
src/pages/tutorials/index.md
              1:1  warning  You are not online and have not set skipOffline: true.                                                                                                                 no-dead-urls  remark-lint

⚠ 512 warnings

I don't know why, but maybe adding the skipOffline option would help. It would be nice to be able to run locally without the noise.

[jeff-matthews]

Commit d8da865 reduced the warning count from 58 to 19.

The output also matches my local test run after adding the skipOffline: "true" option.

All but two of the remaining 19 are located in MRG files, which I'm reluctant to fix since the source comes from READMEs in code base repos. Fixing them here will only be temporary. The test will start failing again when we regenerate the MRG files from source unless we fix the links in the source READMEs.

@dshevtsov, how do you want to proceed for MRG files?

[dshevtsov]

I cannot reproduce the issue with You are not online and have not set skipOffline: true.
I tried both with and without VPN.

As a project owner you are good to go for any changes in the config that work for you.

Any changes to MRG have to be ported to the codebase.

[dshevtsov]

I noticed that running the test with and without VPN returns different results. It appears that when I run the test without VPN, my Internet provider blocks the excessive outgoing connections.

[jeff-matthews]

I re-ran the this job because the symfony site was just running slow and the links finally resolved. Test results confirm.

Only 17 warnings left to resolve (MRG topics).

[jeff-matthews]

Local testing is inconsistent with the latest validation run.

Local testing produces no warnings, but latest validation run produces two warnings. I suspect it has something to do with caching as mentioned in the plugin README:

[check-links](https://github.com/transitive-bullshit/check-links) memoizes results, so on any given run each URL will only be pinged once; subsequent checks will be returned from the cache.

I'm not sure where the cache lives though. Maybe somewhere in the node_modules directory.

Anyway, I'll fix the two remaining warnings and hopefully the validation run will pass.