Most `*.md` files were converted to `*.mdx` in modelcontextprotocol#283, but `package.json`
scripts and CI config were not updated to match.  This commit makes the
necessary updates.

Unfortunately, Prettier currently only supports MDX v1 (see
[prettier#15221][] and [prettier#12209][]), which causes it to treat
content inside JSX tags as JSX instead of Markdown unless the content is
surrounded by blank lines (see [prettier#16589][]).  Therefore, this
commit also adds blank lines inside JSX elements where necessary.

This commit also removes the 89-character line wrap rule for `.md` files
rather than update it for `.mdx` files.  It's not clear that the rule
was helpful, and we already have many files that do not abide by it:

  ```console
  $ grep -l '.{90}' docs/**/*.mdx | wc -l
  59
  ```

For ease of rebasing, this commit _does not_ actually format the files
using Prettier.  That will be done in the next commit.

[prettier#15221]: prettier/prettier#15221
[prettier#12209]: prettier/prettier#12209
[prettier#16589]: prettier/prettier#16589