[DOC] Fix broken links

[PiVortex]

There are some entry points to the docs from browser that need to be fixed. Please add anymore you find. List of broken links:

• [x] https://docs.near.org/sdk/welcome
• [x] https://docs.near.org/sdk/js/testing/integration-tests
• [x] https://docs.near.org/sdk/rust/testing/integration-tests#:~:text=Integration%20tests%20provide%20the%20ability,structured%20execution%20outcomes%2C%20and%20more.
• [x] https://docs.near.org/sdk/rust/testing/unit-tests

[gagdiez]

fixed by adding redirects on Render