Fix some links

[sogaiu]

This PR contains a few suggested changes to links on the "Developing for Pulsar" page.

Clicking on the links for the following text yield 404s here:

• add a syntax theme
• add a UI theme
• add one or more language grammars

It looks like the intended pages exist and slight modifications seem to work:

• add a syntax theme
• add a UI theme
• add one or more language grammars

[DeeDeeG]

Looks good to me, thank you for the contributions!

I tested this locally and the links are indeed fixed for me.

[DeeDeeG]

By the way, if you want the commits to show a link to your username and profile on GItHub.com, you may want to update your git config user.email.

You can do like this git config --global user.email [numbers here][email protected].

(Documentation: https://docs.github.com/en/account-and-profile/setting-up-and-managing-your-personal-account-on-github/managing-email-preferences/setting-your-commit-email-address#about-commit-email-addresses)

[EDIT: I typo'd noreply as norply. My apologies.]

If you don't prefer the commits link to your GitHub username and profile, then no need to worry about it.

[sogaiu]

Thanks so much for the consideration and detailed info!

I was aware of not showing up but I've considered this a bug on GitHub's part but did not know the specifics, TIL!