fix(docs): Better trailing slash omission

[Josmithr]

Partial reversion of #23286.

SWA's implementation of "trailingSlash": "never" is currently flawed. The resulting redirect ends up exposing the underlying SWA URL of the site, rather than our configured front door URL. E.g. https://fluidframework.com/docs/concepts/signals/ ends up redirecting to https://salmon-sand-0b7fa7c1e.1.azurestaticapps.net/docs/concepts/signals.

• See Redirects in trailingSlash configuration do not recognize X-Forwarded-Host correctly Azure/static-web-apps#1036

This PR removes that configuration flag, and instead leverages Docusaurus's trailingSlash configuration option to not emit trailing slashes.

This required updating a handful of relative URL links to use file path links. The guidance documentation in the README has been updated to offer new guidance around relative links in light of new learnings.

AB#25895

[alexvy86]

Good thing you caught it :)

[github-actions]

🔗 No broken links found! ✅

Your attention to detail is admirable.

linkcheck output

> fluid-framework-docs-site@0.0.0 ci:check-links /home/runner/work/FluidFramework/FluidFramework/docs
> start-server-and-test "npm run serve -- --no-open" 3000 check-links

1: starting server using command "npm run serve -- --no-open"
and when url "[ 'http://127.0.0.1:3000' ]" is responding with HTTP status code 200
running tests using command "npm run check-links"


> fluid-framework-docs-site@0.0.0 serve
> docusaurus serve --no-open

[SUCCESS] Serving "build" directory at: http://localhost:3000/

> fluid-framework-docs-site@0.0.0 check-links
> linkcheck http://localhost:3000 --skip-file skipped-urls.txt

Crawling...

Stats:
  170098 links
    1596 destination URLs
    1825 URLs ignored
       0 warnings
       0 errors