Fix "introduction" link, so docs work

[nottrobin]

(Found your project through https://goodfirstissue.dev/, which I was looking at because my team are getting an early start on Hacktoberfest this week. Looks super interesting. Thanks!)

At present, if you click "Introduction" in the README.md it takes you to https://docs.lando.dev/getting-started. If you then read all the way down the page and click the "what-it-do.html" next link at the bottom, it will take you to https://docs.lando.dev/what-it-do.html which is a 404.

Adding a / makes that link take you to https://docs.lando.dev/getting-started/what-it-do.html instead, which works.

This fixes this one instance of this flow from the README.md page. However, a better fix would be to do one of the following:

a) Configure the sever to use redirects to always add the / to the end of directory URLs; or:
b) Use absolute rather than relative links in the documentation navigation so it doesn't matter what the path of the origin page is.

[stale]

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