Add dedicated page for LEKTOR_DEV

[nixjdm]

There's currently two docs pages that mention this env var, and they describe some things it does that are relevant to that section. There's more to say elsewhere though, like enabling the debug jinja extension, or using using the DevTools (which I haven't tried in a minute, for the record.

So a few more existing spots could be extended, but I also think this env var deserves it's own page where everything it does is listed. https://www.getlektor.com/docs/plugins/dev/ doesn't really fit, since that's nested under plugins. Maybe a new guide would be an acceptable fit? https://www.getlektor.com/docs/guides/development/ for general development, in which this env var is given a primary section, with subsections for each feature it enables? Another section could link to the plugin development page, and another to the page for lektor dev shell.