docs testing: Workaround the ratelimitting on Github Links.

[aero31aero]

Recently, we have had quite a lot of failures in CI due to HTTP 429 Too Many Requests error from our github.com/zulip links. After this was reported in # automated testing, we merged a fix: b25e02a.

We should try to figure out a way to properly test GitHub links in CI and then eventually reenable testing github.com/zulip links.

tools/test-documentation is the relevant test suite.

[zulipbot]

Hello @zulip/server-testing members, this issue was labeled with the "area: testing-infrastructure" label, so you may want to check it out!

[timabbott]

We should be able to do some combination of skipping testing of GitHub search links and verifying the paths exist in the repository that we're testing, to fix this.

[timabbott]

git grep BaseDocumentationSpider is helpful for finding the code.