RST backtick refactor manual.rst

[quantimnot]

Added .. default-role:: code directive.

Replaced double backticks with single backticks and realign shifted table columns.

[timotheecour]

• followup rst: single backticks now render correctly in both rst2html and github #17028
• see nim doc/rst2html renders \v, \\, etc incorrectly #17260; how about we wait a bit until we know whether nim doc/rst2html renders \v, \\, etc incorrectly #17260 can be fixed quickly enough so that this PR doesn't need patching
• to view generated html fast, use: nim rst2html -r --lib:lib --doccmd:skip doc/manual.rst

[timotheecour]

@quantimnot how about, for this PR as well as #17258, keep the double backticks when the content contains a \ somewhere inside.

Then we can move forward with this PR without waiting on #17315, and if/when #17315 is merged, we can revisit those small number of cases in subsequent PR.

[timotheecour]

Suggested change

	``\\`` '0'..'9'+ `character with decimal value d`:idx:;
	``\`` '0'..'9'+ `character with decimal value d`:idx:;

[timotheecour]

Suggested change

	``\\`` '0'..'9'+ `character with decimal value d`:idx:;
	``\`` '0'..'9'+ `character with decimal value d`:idx:;

[timotheecour]

Suggested change

	1. `std`: The `std` pseudo directory is the abstract location of Nim's standard
	library. For example, the syntax `import std / strutils` is used to unambiguously
	refer to the standard library's `strutils` module.
	2. `pkg`: The `pkg` pseudo directory is used to unambiguously refer to a Nimble
	package. However, for technical details that lie outside the scope of this document,
	its semantics are: *Use the search path to look for module name but ignore the standard
	library locations*. In other words, it is the opposite of `std`.
	1. `std`: The `std` pseudo directory is the abstract location of Nim's standard
	library. For example, the syntax `import std / strutils` is used to unambiguously
	refer to the standard library's `strutils` module.
	2. `pkg`: The `pkg` pseudo directory is used to unambiguously refer to a Nimble
	package. However, for technical details that lie outside the scope of this document,
	its semantics are: *Use the search path to look for module name but ignore the standard
	library locations*.
	In other words, it is the opposite of `std`.

[timotheecour]

Suggested change

	See the `threads <threads.html>`_ and `channels <channels.html>`_ modules
	See the `threads <threads.html>`_ and `channels <channels_builtin.html>`_ modules

[timotheecour]

LGTM; I've checked thoroughly in 2 different ways:

• checking the diff
• checking the rendered html (nim rst2html) and going page by page carefully, comparing the html before PR and after PR; the pages align and any difference would be obvious to the eye; only required 1 place to offset manually (explained below)
• this PR was rebased but incorrectly dropped the (tiny) diff from 2 recent PR's : rename channels to channels_builtin #17330 and fix RST parsing when no indent after enum.item (fix #17249) #17257 ; this can be fixed up in a followup PR
• also noted \\ vs \ (ie a \ was inserted wrongly) which can be fixed later
• everything else was identical, except for things like:

## ``foo``
=>
## `foo`

in code blocks, which render differently (since its verbatim); but this is an expected, desirable change.

[timotheecour]

I've merged this after a careful review (see previous comment) to avoid this PR from bit-rotting and needing to be rebased again and again; I've noted the followups needed.

[timotheecour]

@quantimnot thanks for doing this! could you please followup with a PR to address remaining points? (can be bundled with the fixups needed for #17258 (comment))