Complete `Concepts` subcategory in docs

[Brendan-Reid1991]

In docs/ there is a concepts/ folder with the following markdown files:

• compilation.md
• data_format.md
• evaluation.md
• symbolics.md

At present only compilation.md is populated, although it is clear that at one point each of these was meant to be written. In index.md a reference is made to "the concepts tab" but upon clicking it there is only one page - the compilation page.

The remaining files should be completed in order to nicely round out the docs.