[repo-assist] Allow sources to be specified as Environment Variables, with docs

[github-actions]

🤖 This PR was created by Repo Assist, an automated AI assistant, in response to @dsyme's request on #4271: "Please make a new PR with the same contents as this PR but also adding docs."

What this does

This re-applies the change from #4271 (originally by @msarilar) on top of current master, and additionally documents the new feature as requested by @forki and @dsyme in that PR's discussion.

Feature (from #4271)

Allows an entire source line in paket.dependencies to be specified via a single environment variable, e.g.:

source %FEED_URL%

where FEED_URL contains everything that would normally follow source (URL plus optional username/password/authtype). This avoids exposing the feed URL itself in source control, addressing #1796.

Docs added

Added a section to docs/content/nuget-dependencies.md explaining the source %FEED_URL% syntax, right after the existing username/password-as-env-var section.

Changes

• src/Paket.Core/Versioning/PackageSources.fs: parse the whole source line from an environment variable when present.
• tests/Paket.Tests/DependenciesFile/ParserSpecs.fs: added tests for source-in-env-var, with and without credentials (ported from Allow sources to be specified as Environment Variables #4271).
• docs/content/nuget-dependencies.md: new documentation section.

Test Status

✅ Build succeeded (dotnet build src/Paket.Core/Paket.Core.fsproj -c Release, net461 + netstandard2.0).
✅ dotnet test tests/Paket.Tests/Paket.Tests.fsproj --filter "FullyQualifiedName~ParserSpecs" -f net9: 154/154 passed, including the two new tests (should read config with source in env variable, should read config with source and credentials in env variable).

Closes the doc gap noted on #4271; that PR can be closed once this is merged, since it targets msarilar/Paket (a fork Repo Assist has no push access to).

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