tcg: convert tcg/README to rst

tcg: convert tcg/README to rst

Convert tcg/README to rst and move it to docs/devel as a new "TCG Intermediate
Representation" page. There are a few minor changes to improve the aesthetic
of the final output which are as follows:

  - Rename the title from "Tiny Code Generator - Fabrice Bellard" to "TCG
    Intermediate Representation"

  - Remove the section numbering

  - Add the missing parameters to the ssadd_vec operations in the "Host
    vector operations" section

  - Change the path to the Atomic Operations document to use a proper
    reference

  - Replace tcg/README in tcg.rst with a proper reference to the new document

Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Reviewed-by: Fabiano Rosas <farosas@suse.de>
Message-Id: <20221130100434.64207-2-mark.cave-ayland@ilande.co.uk>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>