fix: local documentation

[jackwilsdon]

express-static prefers directories over files when using the extensions option, meaning an extensionless route will always route to the directory instead of the file (expressjs/serve-static#138). This causes URLs like http://localhost:9078/docs/docs/configuration to work when navigating from other pages (due to it being a SPA), but on reload it redirects to http://localhost:9078/docs/docs/configuration/ and returns a 404 (note the trailing slash).

This PR re-enables trailing slashes, which turns this structure:

docs
|- configuration
|  |- plex.html
|- configuration.html

Into:

docs
|- configuration
   |- plex
   |  |- index.html
   |- index.html

This fixes local docs (served by the backend).

I've deployed it to GH pages here: https://jackwilsdon.github.io/multi-scrobbler/

All existing (normal) URLs will work - GitHub Pages will just add a trailing slash via a 301 redirect. The only URLs that will no longer work are the ones with extensions (e.g. https://jackwilsdon.github.io/multi-scrobbler/docs/configuration.html), but nothing on the site links to that anyway from what I can tell.

We could only set this option to true for GitHub pages, but it seemed neater to use the same option for both.

Checklist before requesting a review

• I have read the contributing guidelines.

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Describe your changes

Issue number and link, if applicable

[FoxxMD]

Awesome! Thanks for finding and fixing that.