Client specification v2.1 compliance: add placeholder escaping support

[kbdharun]

Client Specification Changelog 📜

• Add a requirement to support escaping the placeholder syntax on certain pages.
• Add suggestion to detect new platforms added to the relevant pages directory automatically.

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IMO we don't need to implement the automatic platform detection suggestion, as we already detect platforms for flags from the definitions. When we need to add new platforms to it we must parallelly add support for Unit tests too to check if it works fine.

So we need to implement the below requirements to support placeholder escaping to comply with v2.1:

Although this specification is about the interface that clients must provide, it is also worth noting that pages are written in standard CommonMark, with the exception of the non-standard {{ and }} placeholder syntax, which surrounds values in an example that users may edit. Clients MAY highlight the placeholders and MUST remove the surrounding curly braces. Clients MUST NOT treat them as the placeholder syntax if they are escaped using \ (i.e. \{\{ and \}\}) and MUST instead display literal braces, without backslashes. Placeholder escaping applies only when both braces are escaped (e.g. in \{ or \{{, backslashes MUST be displayed). Clients MUST NOT break if the page format is changed within the CommonMark specification.

Direct link to section with examples: https://github.com/tldr-pages/tldr/blob/main/CLIENT-SPECIFICATION.md#page-structure