Implement Pandoc Markdown concise link extension (#20304)

* Implement Pandoc Markdown concise link extension

This implements #20127.
Besides reference to headings we also support doing references
to Nim symbols inside Nim modules.

Markdown:
```
Some heading
------------

Ref. [Some heading].
```

Nim:
```
proc someFunction*() ...

... ## Ref. [someFunction]
```

This is substitution for RST syntax like `` `target`_ ``.
All 3 syntax variants of extension from Pandoc Markdown are supported:
`[target]`, `[target][]`, `[description][target]`.

This PR also fixes clashes in existing files, particularly
conflicts with RST footnote feature, which does not work with
this PR (but there is a plan to adopt a popular [Markdown footnote
extension](https://pandoc.org/MANUAL.html#footnotes) to make footnotes work).

Also the PR fixes a bug that Markdown links did not work when `[...]`
section had a line break.

The implementation is straightforward since link resolution did not
change w.r.t. RST implementation, it's almost only about new syntax
addition. The only essential difference is a possibility to add a custom
link description: form `[description][target]` which does not have an
RST equivalent.

* fix nim 1.0 gotcha

src/db_odbc.nim

@@ -168,7 +168,7 @@ proc dbError*(db: var DbConn) {.
     raise e
 
 proc sqlCheck(db: var DbConn, resVal: TSqlSmallInt) {.raises: [DbError]} =
-  ## Wrapper that raises [EDb] if `resVal` is neither SQL_SUCCESS or SQL_NO_DATA
+  ## Wrapper that raises `EDb` if `resVal` is neither SQL_SUCCESS or SQL_NO_DATA
   if resVal notIn [SQL_SUCCESS, SQL_NO_DATA]: dbError(db)
 
 proc sqlGetDBMS(db: var DbConn): string {.
@@ -304,7 +304,7 @@ iterator instantRows*(db: var DbConn, query: SqlQuery,
                       args: varargs[string, `$`]): InstantRow
                 {.tags: [ReadDbEffect, WriteDbEffect].} =
   ## Same as fastRows but returns a handle that can be used to get column text
-  ## on demand using []. Returned handle is valid only within the iterator body.
+  ## on demand using `[]`. Returned handle is valid only within the iterator body.
   var
     rowRes: Row = @[]
     sz: TSqlLen = 0