### Bug description

When linking from a slide deck to a plain HTML-format page using a link like `[link](/index.qmd)` displays as plaintext QMD in the browser. I expected this to display as properly-rendered HTML. Changing the link to `[link](/index.html)` resolves the issue, but break's Quarto's ability to detect broken internal links (e.g. no warnings are thrown if I link to a non-existent file `[link](/foo.html)`)


#### Reproducing from repo

_Reproduction repository: https://github.com/MattF-NSIDC/sscce-quarto-slides-link-to-html-issue_

Clone the repo, run `quarto preview` (or render), and click the link in the slide deck. Your browser URL bar should show `.../index.qmd` instead of `.../index.html` and the content displayed should be plaintext. Expected this to be properly rendered HTML.


#### Reproducing from scratch

1. Create the project

  ```
  quarto create-project --type=website
  ```

2. Create a slides directory with `_metadata.yml` which overrides the format of the
   directory's contents to `revealjs`. 
3. Create a `slides/test.qmd` which has a link to `index.qmd`.
4. Render or preview the site

  ```
  quarto preview
  ```

5. Navigate to the slide deck in the browser, and click the "Go home" link
6. Observe the browser shows plaintext instead of expected HTML, and the URL bar says
   `/slides/test.qmd` instead of the expected `slides/test.html`.
7. The issue can be resolved by editing `slides/test.qmd` to link to `/index.html`
   instead of `/index.qmd`. **NOTE**: This breaks the ability of Quarto to detect broken
   links (e.g. it will be happy with linking `/foo.html` even though that doesn't exist).

### Versions

* RStudio: None
* OS: Ubuntu 20.04
* Quarto: 1.2.335, 1.3.262, 1.3.309

#### Quarto check

```
$ quarto check

[✓] Checking versions of quarto binary dependencies...
      Pandoc version 3.1.1: OK
      Dart Sass version 1.55.0: OK
[✓] Checking versions of quarto dependencies......OK
[✓] Checking Quarto installation......OK
      Version: 1.3.262
      Path: /opt/quarto/bin

[✓] Checking basic markdown render....OK

[✓] Checking Python 3 installation....OK
      Version: 3.8.10
      Path: /usr/bin/python3
      Jupyter: 4.6.3
      Kernels: python3

(/) Checking Jupyter engine render..../usr/lib/python3/dist-packages/requests/__init__.py:89: RequestsDependencyWarning: urllib3 (1.26.14) or chardet (3.0.4) doesn't match a supported version!
  warnings.warn("urllib3 ({}) or chardet ({}) doesn't match a supported "
Traceback (most recent call last):
  File "/opt/quarto/share/jupyter/jupyter.py", line 21, in <module>
    from notebook import notebook_execute, RestartKernel
  File "/opt/quarto/share/jupyter/notebook.py", line 18, in <module>
    from nbclient import NotebookClient
ModuleNotFoundError: No module named 'nbclient'
[✓] Checking Jupyter engine render....OK
```

### Checklist

- [X] Please include a minimal, fully reproducible example in a single .qmd file? Please provide the whole file rather than the snippet you believe is causing the issue.
- [X] Please [format your issue](https://quarto.org/bug-reports.html#formatting-make-githubs-markdown-work-for-us) so it is easier for us to read the bug report.
- [X] Please document the RStudio IDE version you're running (if applicable), by providing the value displayed in the "About RStudio" main menu dialog?
- [X] Please document the operating system you're running. If on Linux, please provide the specific distribution.
- [X] Please provide the output of `quarto check` so we know which version of quarto and its dependencies you're running.
2023-04-05T18:30:03Z
MattF-NSIDC
bug
Future
open
Linking from `revealjs` slides to a plain `html`-format page yields plaintext instead of rendered output in browser
https://github.com/quarto-dev/quarto-cli/issues/5106