Documentation improvement

[qiushiyan]

First, great project! It only took me less than 30 minutes to migrate from contentlayer to velite and it brings lots of improvements.

There are a couple of minor issues in the documentation, including capitalization and grammar mistakes. For example, in
nextjs integration, the second sentence should be here **are** some recipes for help you better integrate Velite with Next.js. and in last modified schema some sentences are not capitalized.

Some wordings can also be improved, e.g.

Considering that Velite's scenario often needs to obtain metadata information about the current file in the schema ...

can be changed to

Since Velite users often need to obtain the current file metadata in the schema ...

In git timestamp the imports of promisify and exec are not shown.

I am not sure if you are actively working on the docs, and if not, is it ok that I make a PR to add doc improvements?

[zce]

PR welcome