Build RTD in one shot

[freakboy3742]

A series of changes to simplify our RTD usage, and resolve issues found during launching the website (see beeware/website#4).

• Modifies the RTD configuration to not lint, moving linting to a CI task. This is on the basis that we don't need to reverify links as part of a deploy - we only need to verify that they're accurate as part of an initial PR. Linting is now done as a CI task, in parallel with the RTD build
• Modifies RTD builds to be a single "docs-all". This means that we take responsibility for generating all the languages, in the right directory (i.e., /html/index.html is the English index; /html/de/index.html is German).
• Modifies the language selector so that en points at the root path.
• Modifies the language selector so that picking a language goes to the same page on the chosen language.

This means:

1. We only need a single RTD project for each documentation target, rather than one per language. This massively improves our concurrency problems
2. We can use versioning if we want to.
3. We aren't blocked on linting problems on re-deploys
4. If we choose to show the RTD flyout menu, the RTD configuration will show the version selector if the project has versions - but won't show the language.

For review purposes, I've modified the RTD configuration for docs-tools to not have any translations, and be a "versioned, no languages" configuration. That means docs-tools-fr and docs-tools-de aren't meaningful tests for this PR. Linting fails because the DE and FR sites don't have the same URL structure any more - it's latest/de, not de/latest.

PR Checklist:

• All new features have been tested
• All new features have been documented
• I have read the CONTRIBUTING.md file
• I will abide by the code of conduct

[kattni]

Site build appears to be successful.