Integrate ADOPTERS.md in docs

[twz123]

Description

• Move ADOPTERS file into the docs folder, so that it becomes a part of the rendered documentation.
• Re-add an ADOPTERS.md in the project root as a symlink to docs/adopters.md, so that it shows up in the project root, as well.
• Add the adopters to the docs navigation

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update

How Has This Been Tested?

• Manual test
• Auto test added

Checklist:

• My code follows the style guidelines of this project
• My commit messages are signed-off
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules
• I have checked my code and corrected any misspellings

[jnummelin]

LGTM