Replace few Tact code snippets with Tolk

[novusnota]

Currently, there are only four files with Tact code blocks:

• techniques/contract-sharding.mdx
• ecosystem/blueprint/coverage.mdx
• ton/proofs/basic-proof-concepts.mdx
• tvm/exit-codes.mdx

First three have only one snippet, where the "Exit codes" page has around 13.

(created when working on #726)

[anton-trunov]

@novusnota let's split this issue into several ones and add those to the corresponding places in the backlog tree

see, for instance, #744