[content] Revisit static html serving options

https://github.com/adobe/helix-cli/pull/2689 is going to introduce `aem content clone` which downloads the content locally in the `./content` folder (default naming convention) and of course, the server serves the content from this folder with priority.

At the same time, https://github.com/adobe/helix-cli/pull/2697 introduced a new parameter `--html-mount` which works with the already existing `--html-folder` parameter:
- `--html-folder` option enables serving HTML files without extensions, useful for previewing content changes when you don't have access to the authoring system.
- use `--html-mount` to control the URL path where files are served. Without it, files are served at `/FOLDER/*`.

We now have 2 concepts doing something similar and potentially conflicting.

My proposal:
- get rid of `--html-folder` and use `./content` as convention and default value
- if there is a real need, rename `--html-mount` to `--content-mount` that would define a different root path (default is `/`). I do not see a scenario where a different root path might be needed (urls not aligned with content structure ?)

cc @alexcarol @tripodsan @davidnuescheler

Start by comparing PRs #2689 and #2697 and locating the CLI entry points for --html-folder and --html-mount. Determine whether the ./content convention can replace the existing option without breaking URL behavior; done means a decided, consistent serving model and aligned option handling.

Written by the indexing model from the issue text.