docs/tour: make ready for go-live

[jpluscplusm]

The existing tour will be elevated temporarily whilst the Language Guide is hidden for go-live.

• [x] add any pages required by the temporary removal of the LG
• [x] add any pages required to act as inbound link targets, where existing pages currently link to the LG
• [x] do a tone-and-language edit with a focus on friendliness, making it suitable to be the introductory pass through the language until the LG lands after go-live
  • remove the result.txt header visible on the RHS of all {{{with code}}} blocks
  • where appropriate, turn the code blocks' RHS into a TERMINAL block
• [x] add a front page for the tour, in particular to introduce its different sections for the CUE newcomer who arrived from docs/introduction or the front page
• [x] fix up the flow through the different sections
  • add a front page for each section
  • OR merge the sections into 1 single flow
  • remove the auto-generated index on each section's front page
• [ ] add a section demonstrating the CLI's core operations
  • include core features demonstrated on docs/concept/{json,yaml}

[jpluscplusm]

• add any pages required to act as inbound link targets, where existing pages currently link to the LG

After a review of links to the language guide, and the tour's current set of pages, I conclude that these pages would need adding to the existing tour to meet the needs of the links that currently exist elsewhere on the site:

• Fields: docs/language-guide/data/maps-and-fields
  • place before https://alpha.cuelang.org/docs/tour/basics/duplicate-fields/ ?
  • but "structs" are here: https://alpha.cuelang.org/docs/tour/types/optional/
• Optional field constraints: docs/language-guide/schemas-and-validation/field-constraints#optional-field-constraints
  • already covered on https://alpha.cuelang.org/docs/tour/types/optional but it's not explicitly called out, except in the code example
• Required field constraints: docs/language-guide/schemas-and-validation/field-constraints#required-field-constraints
  • add to docs/tour/types/optional ?
  • rename /optional to /structs ?
• Regular fields: docs/language-guide/data/maps-and-fields#field-names
• Value constraints: docs/language-guide/schemas-and-validation/value-constraints
• Querying: language-guide/templating/comprehensions
  • does this actually need adding to the tour?
  • It's only linked to (or mentioned as "querying") in a single place on the site (/docs).

[jpluscplusm]

• Fields: docs/language-guide/data/maps-and-fields
• Optional field constraints: docs/language-guide/schemas-and-validation/field-constraints#optional-field-constraints
• Required field constraints: docs/language-guide/schemas-and-validation/field-constraints#required-field-constraints
• Regular fields: docs/language-guide/data/maps-and-fields#field-names

Covered by changes proposed in https://cuelang.org/cl/1177590

• Value constraints: docs/language-guide/schemas-and-validation/value-constraints

Covered by changes proposed in https://cuelang.org/cl/1177605

• Querying: language-guide/templating/comprehensions

Covered by changes proposed in https://cuelang.org/cl/1177561

[jpluscplusm]

• add any pages required by the temporary removal of the LG

Let's add docs/tour/expressions/query-projection/, after https://alpha.cuelang.org/docs/tour/expressions/conditional/, that shows an example of:

• a pile of data (trying to avoid a separate data file, so the tour doesn't gain its first code block with multiple files in the same 2x2 grid position; but not trying so hard it distracts)
• a _query field that limits the data items somehow
• a result struct containing all the items from _query, but with only a subset of their fields.

[jpluscplusm]

Let's add docs/tour/expressions/query-projection/

Proposed in https://cuelang.org/cl/1177629

[jpluscplusm]

• fix up the flow through the different sections

https://review.gerrithub.io/c/cue-lang/cuelang.org/+/1177973

[jpluscplusm]

@myitcv Before I start writing it, do you agree with my final (added later) bullet point; that the tour should include "a section demonstrating the CLI's core operations"?

[jpluscplusm]

• add a front page for the tour

https://cuelang.org/cl/1178168

[jpluscplusm]

Closed via https://github.com/cue-lang/cuelang.org/commit/ff7f9290934b1cfb133709506d2a47a48ce7a5a7.