docs: Installation guide should mention Positron and recommend TOML extension

[nealrichardson]

https://github.com/posit-dev/publisher/blob/main/docs/installation.md doesn't mention Positron or Open VSX, and it should.

It should also recommend installing the even better TOML extension, which index.md mentions in passing.

The file should also generally be reworked to look like the install page for published software and not a dev project, i.e. heavily de-emphasize the manual installation routes. install-publisher.bash and all that feels like a vestige from before the publisher was released to the marketplaces.