Isolate config in individual docs pages for sclicheck.DocTests

[Gedochao]

Describe what needs to be done and why
sclicheck currently isolates any config sub-command calls within docs-tests.
This prevents the tested settings to leak to other parts of CI builds & tests, but unfortunately does not prevent those settings to leak between individual docs pages tests.
i.e. sclicheck.DocTests.guide proxies currently doesn't run its examples, as the proxy settings would be shared with other docs tests and break them (as proxy settings are shared with coursier).

Ideally, config should be isolated to individual docs' pages tests.

Is your feature request related to a past ticket or discussion?
#1917 (11c56c4 in particular)

Describe alternatives you've considered
We could alternatively isolate each bash code block's config, but that would disable testing the whole flow described in a doc.