Use 1-indexed and non-padded tutorial URLs

[aomarks]

Previously, we used 0-indexed and left-padded tutorial URLs. For example, the 2nd step of a tutorial had URL /tutorials/<name>/#01.

Now it is 1-indexed, and not padded. For example: /tutorials/<name>/#2.

I think 0-indexed is more intuitive, because it means the URLs are aligned with the "Step 2/8" text on the page.

I think left-padding was unnecessary in URLs, plus by removing it, we have a convenient way to detect people still on the 0-indexed scheme, and fix the URLs to be 1-indexed. This way nobody will get jumped ahead if they reload before/after the new scheme is deployed.

Also fixes a bug where we would set the URL to /tutorials/<name>#2 (no trailing slash), but then after reloading it would turn into /tutorials/<name>/#2. Now we always set the final form.

[github-actions]

A live preview of this PR will be available at the URL(s) below.
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https://pr832-a23a0a4---lit-dev-5ftespv5na-uc.a.run.app/

[AndrewJakubowicz]

Nice! Also tested manually in the preview url. Went over the word-viewer and async directive checking that solve and all the steps were working.

Nice!