[docs] [issue35054] Add more index entries for symbols

[Serhiy Storchaka]

New submission from Serhiy Storchaka <storchaka+cpython at gmail.com>:

The proposed PR adds index entries for all operators, syntax elements and keywords. It also adds index entries for symbols in related minilanguages:

* escape sequences in string literals
* metacharacters and escape sequences in regular expressions
* symbols in printf-style formatting, string formatting, f-strings and template strings
* special symbols for OS (pathnames and interpolation)
* special symbols in doctests
* and others

Also fixed several errors in formatting.

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assignee: docs at python
components: Documentation
messages: 328329
nosy: docs at python, eric.araujo, ezio.melotti, georg.brandl, serhiy.storchaka, willingc
priority: normal
severity: normal
status: open
title: Add more index entries for symbols
type: enhancement
versions: Python 3.6, Python 3.7, Python 3.8

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue35054>
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