Document the environment variables that can affect the behavior of PyGMT

[seisman]

This PR adds another page to the "Technical Reference" section. On this page, the environmental variables that PyGMT uses are documented. There are still many other variables that are not documented since they are not explicitly used by PyGMT (e.g., HOME, GMT_SHAREDIR). We may document them later.

Preview:

• https://pygmt-dev--3432.org.readthedocs.build/en/3432/techref/environment_variables.html
• https://pygmt-dev--3432.org.readthedocs.build/en/3432/api/generated/pygmt.Figure.show.html
• https://pygmt-dev--3432.org.readthedocs.build/en/3432/api/generated/pygmt.Figure.timestamp.html
• https://pygmt-dev--3432.org.readthedocs.build/en/3432/api/generated/pygmt.x2sys_init.html
• https://pygmt-dev--3432.org.readthedocs.build/en/3432/api/generated/pygmt.x2sys_cross.html

[seisman]

will merge in 24 hours if no further comments.