feat(selenium-migration): rename Selenium.Axe to Deque.AxeCore.Selenium

[dbjorge]

This PR updates all references to the Selenium.Axe project naming to instead use Deque.AxeCore.Selenium naming/layout. Specifically:

• It moves the source files out of directories named after Selenium.Axe and into flatter src and test directories, similar to the layout used for the commons package in feat(commons): set up empty commons projects #27
• It updates the namespace used by all source files from Selenium.Axe to Deque.AxeCore.Selenium
• It updates the .csproj files' names and contents to match the new naming
• It migrates from Selenium.Axe's docsify setup to embedding docs in the package-level README, similar to other comparable Deque integration libraries
• It updates the README and src csproj with the LICENSE/NOTICE info already introduced in previous PRs
• It consolidates the necessary parts of the old .gitignore into the root-level one
• It removes several obsolete files that will be handled at the root level

Out of scope:

• I left the /packages/selenium/.github directory in place for now; we'll migrate the existing contents out of it separately as part of Create/Configure CI pipeline #4 (for the workflows) and [Common] Set up repo-level doc files #7 (for the issue/PR templates)
• For now, I just updated all the namespaces in the existing source files and left them in their current locations. Separate future PRs will move many of them to the commons package and remove several of them related to HTML reporting

Recommend that reviewers verify that:

• dotnet build from the new /packages/selenium/src directory succeeds and produces a nupkg in the expected layout
• dotnet test from the new /packages/selenium/test directory runs and passes all the original tests (note: you will need both a .NET Core 3.1 SDK and a .NET 6 SDK installed)

Completes #11

[AdnoC]

lgtm